Fix T101501: Masks are not visible in Image Editor

Need to initialize the mask drawing overlays when the new space
is created. Otherwise the new space is configured in a way that
the splines are not visible and overlay opacity is 0.

This change fixes the new masking files created. The currently
saved ones need a manual tweak.

source/blender/editors/space_image/space_image.c

@@ -5,6 +5,7 @@
  * \ingroup spimage
  */
 
+#include "DNA_defaults.h"
 #include "DNA_gpencil_types.h"
 #include "DNA_image_types.h"
 #include "DNA_mask_types.h"
@@ -117,6 +118,8 @@ static SpaceLink *image_create(const ScrArea *UNUSED(area), const Scene *UNUSED(
   simage->custom_grid_subdiv[0] = 10;
   simage->custom_grid_subdiv[1] = 10;
 
+  simage->mask_info = *DNA_struct_default_get(MaskSpaceInfo);
+
   /* header */
   region = MEM_callocN(sizeof(ARegion), "header for image");