### 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)

for (KeyValue<Vector2i, TileAlternativesData> E_tile : tiles) {
		for (KeyValue<int, TileData *> E_alternative : E_tile.value.alternatives) {
			E_alternative.value->remove_custom_data_layer(p_index);
		}
	}
}
void TileSetAtlasSource::reset_state() {
	tile_set = nullptr;
	for (KeyValue<Vector2i, TileAlternativesData> &E_tile : tiles) {
		for (const KeyValue<int, TileData *> &E_tile_data : E_tile.value.alternatives) {
			memdelete(E_tile_data.value);
		}
	}
	_coords_mapping_cache.clear();
	tiles.clear();
	tiles_ids.clear();
	_queue_update_padded_texture();
}
void TileSetAtlasSource::set_texture(Ref<Texture2D> p_texture) {
	if (texture.is_valid()) {
		texture->disconnect_changed(callable_mp(this, &TileSetAtlasSource::_queue_update_padded_texture));
	}
	texture = p_texture;
	if (texture.is_valid()) {
		texture->connect_changed(callable_mp(this, &TileSetAtlasSource::_queue_update_padded_texture));
	}
	_queue_update_padded_texture();
	emit_changed();
}
Ref<Texture2D> TileSetAtlasSource::get_texture() const {