After converting sprite image type, 'extras' are regenerated.

src/commands/cmd_change_image_type.cpp

@@ -105,6 +105,12 @@ void ChangeImageTypeCommand::execute(Context* context)
     CurrentSpriteRgbMap rgbmap;
     Undoable undoable(sprite, "Color Mode Change");
     undoable.set_imgtype(m_imgtype, m_dithering);
+
+    // Regenerate extras
+    sprite->prepareExtra();
+    image_clear(sprite->getExtras(),
+		sprite->getExtras()->mask_color);
+
     undoable.commit();
   }
   app_refresh_screen(sprite);

src/raster/undo.cpp

@@ -1546,6 +1546,12 @@ static void chunk_set_imgtype_invert(UndoStream* stream, UndoChunkSetImgType *ch
   if (sprite) {
     chunk_set_imgtype_new(stream, sprite);
     sprite->setImgType(chunk->imgtype);
+
+    // Regenerate extras
+    sprite->prepareExtra();
+    image_clear(sprite->getExtras(),
+		sprite->getExtras()->mask_color);
+
   }
 }