### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

ERR_FAIL_COND_V(!scenes.has(p_id), Ref<PackedScene>());
	return scenes[p_id].scene;
}
void TileSetScenesCollectionSource::set_scene_tile_display_placeholder(int p_id, bool p_display_placeholder) {
	ERR_FAIL_COND(!scenes.has(p_id));
	scenes[p_id].display_placeholder = p_display_placeholder;
	emit_signal(SNAME("changed"));
}
bool TileSetScenesCollectionSource::get_scene_tile_display_placeholder(int p_id) const {
	ERR_FAIL_COND_V(!scenes.has(p_id), false);
	return scenes[p_id].display_placeholder;
}
void TileSetScenesCollectionSource::remove_scene_tile(int p_id) {