### 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_MSG(!sources.has(p_source_id), vformat("Cannot remove TileSet atlas source. No tileset atlas source with id %d.", p_source_id));
	sources[p_source_id]->disconnect_changed(callable_mp(this, &TileSet::_source_changed));
	sources[p_source_id]->set_tile_set(nullptr);
	sources.erase(p_source_id);
	source_ids.erase(p_source_id);
	source_ids.sort();
	terrains_cache_dirty = true;
	emit_changed();
}
void TileSet::remove_source_ptr(TileSetSource *p_tile_set_source) {
	for (const KeyValue<int, Ref<TileSetSource>> &kv : sources) {
		if (kv.value.ptr() == p_tile_set_source) {
			remove_source(kv.key);
			return;
		}
	}
	ERR_FAIL_MSG(vformat("Attempting to remove source from a tileset, but the tileset doesn't have it: %s", p_tile_set_source));
}
void TileSet::set_source_id(int p_source_id, int p_new_source_id) {