### 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)

tile_shape = p_shape;
	for (KeyValue<int, Ref<TileSetSource>> &E_source : sources) {
		E_source.value->notify_tile_data_properties_should_change();
	}
	terrain_bits_meshes_dirty = true;
	tile_meshes_dirty = true;
	notify_property_list_changed();
	emit_changed();
}
TileSet::TileShape TileSet::get_tile_shape() const {
	return tile_shape;
}
void TileSet::set_tile_layout(TileSet::TileLayout p_layout) {