Fix #137821: Crash with grease pencil render, compositor and render region

When using the compositor, the parent scene overrides the render resolution
and region of the child scene. Unlike workbench and EEVEE, the grease pencil
engine was reading this information from the child scene.

Pull Request: https://projects.blender.org/blender/blender/pulls/137923

source/blender/draw/engines/gpencil/gpencil_render.cc

@@ -30,7 +30,6 @@ void GPENCIL_render_init(GPENCIL_Data *vedata,
   GPENCIL_FramebufferList *fbl = vedata->fbl;
   GPENCIL_TextureList *txl = vedata->txl;
 
-  Scene *scene = DEG_get_evaluated_scene(depsgraph);
   const float *viewport_size = DRW_viewport_size_get();
   const int size[2] = {int(viewport_size[0]), int(viewport_size[1])};
 
@@ -84,7 +83,8 @@ void GPENCIL_render_init(GPENCIL_Data *vedata,
     }
   }
 
-  const bool do_region = (scene->r.mode & R_BORDER) != 0;
+  const bool do_region = !(rect->xmin == 0 && rect->ymin == 0 && rect->xmax == size[0] &&
+                           rect->ymax == size[1]);
   const bool do_clear_z = !pix_z || do_region;
   const bool do_clear_col = !pix_col || do_region;