Fix #135673: Grease Pencil: Dash modifier deletes unfiltered strokes

Grease Pencil dash modifier wasn't merging strokes from strokes that are
unfiltered, resulted in only filtered strokes being shown afterwards.
This fix joins generated strokes with what's left behind so there's no
stroke missing.

Pull Request: https://projects.blender.org/blender/blender/pulls/135705

source/blender/modifiers/intern/MOD_grease_pencil_dash.cc

@@ -38,6 +38,8 @@
 #include "MOD_grease_pencil_util.hh"
 #include "MOD_ui_common.hh"
 
+#include "GEO_join_geometries.hh"
+
 namespace blender {
 
 static void init_data(ModifierData *md)
@@ -342,8 +344,20 @@ static void modify_drawing(const GreasePencilDashModifierData &dmd,
   IndexMaskMemory curve_mask_memory;
   const IndexMask curves_mask = modifier::greasepencil::get_filtered_stroke_mask(
       ctx.object, src_curves, dmd.influence, curve_mask_memory);
+  const IndexMask unselected_mask = curves_mask.complement(src_curves.curves_range(),
+                                                           curve_mask_memory);
+  bke::CurvesGeometry unselected_curves = bke::curves_copy_curve_selection(
+      src_curves, unselected_mask, {});
 
-  drawing.strokes_for_write() = create_dashes(pattern_info, src_curves, curves_mask);
+  bke::CurvesGeometry dashed_curves = create_dashes(pattern_info, src_curves, curves_mask);
+
+  Curves *masked_curves_id = bke::curves_new_nomain(dashed_curves);
+  Curves *unselected_curves_id = bke::curves_new_nomain(unselected_curves);
+  bke::GeometrySet masked_geo = bke::GeometrySet::from_curves(masked_curves_id);
+  bke::GeometrySet unselected_geo = bke::GeometrySet::from_curves(unselected_curves_id);
+  bke::GeometrySet joined_geo = geometry::join_geometries({unselected_geo, masked_geo}, {});
+
+  drawing.strokes_for_write() = std::move(joined_geo.get_curves_for_write()->geometry.wrap());
   drawing.tag_topology_changed();
 }