Fix #123220: Export GP SVG/PDF crash when active object is not GP

`GpencilIOParams.ob` is set to `CTX_data_active_object` at the beginning
of the export [which is not ensured to be a valid `OB_GPENCIL_LEGACY`
object, non-valid objects are filtered out later in
`create_object_list`]. From this (potential non-greasepencil) object's
data, a "global `bGPdata` is stored in `GpencilIOParams`.

From here, it can only go downwards as `stroke_point_radius_get`,
`export_stroke_to_polyline`, `BKE_gpencil_stroke_perimeter_from_view`
all use this "global" `bGPdata`.

Two possible solutions might exist:
- [1] just cancel the operator if the active object is not a
`OB_GPENCIL_LEGACY` object
- [2] make sure we use corresponding `bGPdata` when iterating over
valid objects to export

This PR chooses [2] since it also seems wrong to always use the
`bGPdata` from the active object for certain calculations when iterating
many objects. For example, the export ignores different values for
`Thickness Scale` on different objects and always takes the value from
the active object, which does not seem to be by design.

NOTE: this changes behavior (see above) but for the better
Pull Request: https://projects.blender.org/blender/blender/pulls/123224

source/blender/io/gpencil/intern/gpencil_io_base.cc

@@ -251,14 +251,14 @@ float2 GpencilIO::gpencil_3D_point_to_2D(const float3 co)
   return result;
 }
 
-float GpencilIO::stroke_point_radius_get(bGPDlayer *gpl, bGPDstroke *gps)
+float GpencilIO::stroke_point_radius_get(bGPdata *gpd, bGPDlayer *gpl, bGPDstroke *gps)
 {
   bGPDspoint *pt = &gps->points[0];
   const float2 screen_co = gpencil_3D_point_to_2D(&pt->x);
 
   /* Radius. */
   bGPDstroke *gps_perimeter = BKE_gpencil_stroke_perimeter_from_view(
-      rv3d_->viewmat, gpd_, gpl, gps, 3, diff_mat_.ptr(), 0.0f);
+      rv3d_->viewmat, gpd, gpl, gps, 3, diff_mat_.ptr(), 0.0f);
 
   pt = &gps_perimeter->points[0];
   const float2 screen_ex = gpencil_3D_point_to_2D(&pt->x);

source/blender/io/gpencil/intern/gpencil_io_base.hh

@@ -79,7 +79,7 @@ class GpencilIO {
   float2 gpencil_3D_point_to_2D(const float3 co);
 
   /** Get radius of point. */
-  float stroke_point_radius_get(bGPDlayer *gpl, bGPDstroke *gps);
+  float stroke_point_radius_get(bGPdata *gpd, bGPDlayer *gpl, bGPDstroke *gps);
   /** Create a list of selected objects sorted from back to front */
   void create_object_list();
 

source/blender/io/gpencil/intern/gpencil_io_export_pdf.cc

@@ -192,24 +192,24 @@ void GpencilExporterPDF::export_gpencil_layers()
         /* Fill. */
         if ((is_fill) && (params_.flag & GP_EXPORT_FILL)) {
           /* Fill is exported as polygon for fill and stroke in a different shape. */
-          export_stroke_to_polyline(gpl, gps_duplicate, is_stroke, true, false);
+          export_stroke_to_polyline(gpd_eval, gpl, gps_duplicate, is_stroke, true, false);
         }
 
         /* Stroke. */
         if (is_stroke) {
           if (is_normalized) {
-            export_stroke_to_polyline(gpl, gps_duplicate, is_stroke, false, true);
+            export_stroke_to_polyline(gpd_eval, gpl, gps_duplicate, is_stroke, false, true);
           }
           else {
             bGPDstroke *gps_perimeter = BKE_gpencil_stroke_perimeter_from_view(
-                rv3d_->viewmat, gpd_, gpl, gps_duplicate, 3, diff_mat_.ptr(), 0.0f);
+                rv3d_->viewmat, gpd_eval, gpl, gps_duplicate, 3, diff_mat_.ptr(), 0.0f);
 
             /* Sample stroke. */
             if (params_.stroke_sample > 0.0f) {
               BKE_gpencil_stroke_sample(gpd_eval, gps_perimeter, params_.stroke_sample, false, 0);
             }
 
-            export_stroke_to_polyline(gpl, gps_perimeter, is_stroke, false, false);
+            export_stroke_to_polyline(gpd_eval, gpl, gps_perimeter, is_stroke, false, false);
 
             BKE_gpencil_free_stroke(gps_perimeter);
           }
@@ -220,7 +220,8 @@ void GpencilExporterPDF::export_gpencil_layers()
   }
 }
 
-void GpencilExporterPDF::export_stroke_to_polyline(bGPDlayer *gpl,
+void GpencilExporterPDF::export_stroke_to_polyline(bGPdata *gpd,
+                                                   bGPDlayer *gpl,
                                                    bGPDstroke *gps,
                                                    const bool is_stroke,
                                                    const bool do_fill,
@@ -238,7 +239,7 @@ void GpencilExporterPDF::export_stroke_to_polyline(bGPDlayer *gpl,
   copy_v3_v3(&pt_dst->x, &pt_src->x);
   pt_dst->pressure = avg_pressure;
 
-  const float radius = stroke_point_radius_get(gpl, gps_temp);
+  const float radius = stroke_point_radius_get(gpd, gpl, gps_temp);
 
   BKE_gpencil_free_stroke(gps_temp);
 

source/blender/io/gpencil/intern/gpencil_io_export_pdf.hh

@@ -47,7 +47,7 @@ class GpencilExporterPDF : public GpencilExporter {
    * \param do_fill: True if the stroke is only fill
    */
   void export_stroke_to_polyline(
-      bGPDlayer *gpl, bGPDstroke *gps, bool is_stroke, bool do_fill, bool normalize);
+      bGPdata *gpd, bGPDlayer *gpl, bGPDstroke *gps, bool is_stroke, bool do_fill, bool normalize);
   /**
    * Set color.
    * \param do_fill: True if the stroke is only fill.

source/blender/io/gpencil/intern/gpencil_io_export_svg.cc

@@ -213,17 +213,17 @@ void GpencilExporterSVG::export_gpencil_layers()
         if ((is_fill) && (params_.flag & GP_EXPORT_FILL)) {
           /* Fill is always exported as polygon because the stroke of the fill is done
            * in a different SVG command. */
-          export_stroke_to_polyline(gpl, gps_duplicate, node_gpl, is_stroke, true);
+          export_stroke_to_polyline(gpd_eval, gpl, gps_duplicate, node_gpl, is_stroke, true);
         }
 
         /* Stroke. */
         if (is_stroke) {
           if (is_normalized) {
-            export_stroke_to_polyline(gpl, gps_duplicate, node_gpl, is_stroke, false);
+            export_stroke_to_polyline(gpd_eval, gpl, gps_duplicate, node_gpl, is_stroke, false);
           }
           else {
             bGPDstroke *gps_perimeter = BKE_gpencil_stroke_perimeter_from_view(
-                rv3d_->viewmat, gpd_, gpl, gps_duplicate, 3, diff_mat_.ptr(), 0.0f);
+                rv3d_->viewmat, gpd_eval, gpl, gps_duplicate, 3, diff_mat_.ptr(), 0.0f);
 
             /* Sample stroke. */
             if (params_.stroke_sample > 0.0f) {
@@ -286,7 +286,8 @@ void GpencilExporterSVG::export_stroke_to_path(bGPDlayer *gpl,
   node_gps.append_attribute("d").set_value(txt.c_str());
 }
 
-void GpencilExporterSVG::export_stroke_to_polyline(bGPDlayer *gpl,
+void GpencilExporterSVG::export_stroke_to_polyline(bGPdata *gpd,
+                                                   bGPDlayer *gpl,
                                                    bGPDstroke *gps,
                                                    pugi::xml_node node_gpl,
                                                    const bool is_stroke,
@@ -304,7 +305,7 @@ void GpencilExporterSVG::export_stroke_to_polyline(bGPDlayer *gpl,
   copy_v3_v3(&pt_dst->x, &pt_src->x);
   pt_dst->pressure = avg_pressure;
 
-  const float radius = stroke_point_radius_get(gpl, gps_temp);
+  const float radius = stroke_point_radius_get(gpd, gpl, gps_temp);
 
   BKE_gpencil_free_stroke(gps_temp);
 

source/blender/io/gpencil/intern/gpencil_io_export_svg.hh

@@ -88,8 +88,12 @@ class GpencilExporterSVG : public GpencilExporter {
    * \param node_gpl: Node of the layer.
    * \param do_fill: True if the stroke is only fill
    */
-  void export_stroke_to_polyline(
-      bGPDlayer *gpl, bGPDstroke *gps, pugi::xml_node node_gpl, bool is_stroke, bool do_fill);
+  void export_stroke_to_polyline(bGPdata *gpd,
+                                 bGPDlayer *gpl,
+                                 bGPDstroke *gps,
+                                 pugi::xml_node node_gpl,
+                                 bool is_stroke,
+                                 bool do_fill);
 
   /**
    * Set color SVG string for stroke