### 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)

Array output;
	for (const KeyValue<Array, Array> &E : coords_level_proxies) {
		Array proxy;
		proxy.append_array(E.key);
		proxy.append_array(E.value);
		output.push_back(proxy);
	}
	return output;
}
Array TileSet::get_alternative_level_tile_proxies() const {
	Array output;
	for (const KeyValue<Array, Array> &E : alternative_level_proxies) {
		Array proxy;
		proxy.append_array(E.key);
		proxy.append_array(E.value);
		output.push_back(proxy);
	}
	return output;
}
Array TileSet::map_tile_proxy(int p_source_from, Vector2i p_coords_from, int p_alternative_from) const {
	Array from;
	from.push_back(p_source_from);
	from.push_back(p_coords_from);
	from.push_back(p_alternative_from);
	
	if (has_source(p_source_from)) {
		Ref<TileSetSource> source = get_source(p_source_from);