AssetLib: convert video thumbnails to RGBA

Image::blend_rect only works with images that have the same format.

editor/plugins/asset_library_editor_plugin.cpp

@@ -177,6 +177,8 @@ void EditorAssetLibraryItemDescription::set_image(int p_type, int p_index, const
 						thumbnail = thumbnail->duplicate();
 						Point2 overlay_pos = Point2((thumbnail->get_width() - overlay->get_width()) / 2, (thumbnail->get_height() - overlay->get_height()) / 2);
 
+						// Overlay and thumbnail need the same format for `blend_rect` to work.
+						thumbnail->convert(Image::FORMAT_RGBA8);
 						thumbnail->lock();
 						thumbnail->blend_rect(overlay, overlay->get_used_rect(), overlay_pos);
 						thumbnail->unlock();