### 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_INDEX_V(p_index, source_ids.size(), TileSet::INVALID_SOURCE);
	return source_ids[p_index];
}
void TileSet::set_uv_clipping(bool p_uv_clipping) {
	if (uv_clipping == p_uv_clipping) {
		return;
	}
	uv_clipping = p_uv_clipping;
	emit_changed();
}
bool TileSet::is_uv_clipping() const {
	return uv_clipping;
};
int TileSet::get_occlusion_layers_count() const {
	return occlusion_layers.size();
};
void TileSet::add_occlusion_layer(int p_index) {
	if (p_index < 0) {
		p_index = occlusion_layers.size();
	}
	ERR_FAIL_INDEX(p_index, occlusion_layers.size() + 1);
	occlusion_layers.insert(p_index, OcclusionLayer());
	for (KeyValue<int, Ref<TileSetSource>> source : sources) {
		source.value->add_occlusion_layer(p_index);
	}
	notify_property_list_changed();
	emit_changed();
}
void TileSet::move_occlusion_layer(int p_from_index, int p_to_pos) {