### 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_layer_id, custom_data_layers.size(), "");
	return custom_data_layers[p_layer_id].name;
}
void TileSet::set_custom_data_layer_type(int p_layer_id, Variant::Type p_value) {
	ERR_FAIL_INDEX(p_layer_id, custom_data_layers.size());
	custom_data_layers.write[p_layer_id].type = p_value;
	for (KeyValue<int, Ref<TileSetSource>> &E_source : sources) {
		E_source.value->notify_tile_data_properties_should_change();
	}
	emit_changed();
}
Variant::Type TileSet::get_custom_data_layer_type(int p_layer_id) const {
	ERR_FAIL_INDEX_V(p_layer_id, custom_data_layers.size(), Variant::NIL);
	return custom_data_layers[p_layer_id].type;
}
void TileSet::set_source_level_tile_proxy(int p_source_from, int p_source_to) {
	ERR_FAIL_COND(p_source_from == TileSet::INVALID_SOURCE || p_source_to == TileSet::INVALID_SOURCE);
	source_level_proxies[p_source_from] = p_source_to;
	emit_changed();
}
int TileSet::get_source_level_tile_proxy(int p_source_from) {
	ERR_FAIL_COND_V(!source_level_proxies.has(p_source_from), TileSet::INVALID_SOURCE);
	return source_level_proxies[p_source_from];
}
bool TileSet::has_source_level_tile_proxy(int p_source_from) {
	return source_level_proxies.has(p_source_from);
}
void TileSet::remove_source_level_tile_proxy(int p_source_from) {