### 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, physics.size(), Vector<Vector2>());
	ERR_FAIL_INDEX_V(p_polygon_index, physics[p_layer_id].polygons.size(), Vector<Vector2>());
	return physics[p_layer_id].polygons[p_polygon_index].polygon;
}
void TileData::set_collision_polygon_one_way(int p_layer_id, int p_polygon_index, bool p_one_way) {
	ERR_FAIL_INDEX(p_layer_id, physics.size());
	ERR_FAIL_INDEX(p_polygon_index, physics[p_layer_id].polygons.size());
	physics.write[p_layer_id].polygons.write[p_polygon_index].one_way = p_one_way;
	emit_signal(SNAME("changed"));
}
bool TileData::is_collision_polygon_one_way(int p_layer_id, int p_polygon_index) const {
	ERR_FAIL_INDEX_V(p_layer_id, physics.size(), false);
	ERR_FAIL_INDEX_V(p_polygon_index, physics[p_layer_id].polygons.size(), false);
	return physics[p_layer_id].polygons[p_polygon_index].one_way;
}
void TileData::set_collision_polygon_one_way_margin(int p_layer_id, int p_polygon_index, float p_one_way_margin) {
	ERR_FAIL_INDEX(p_layer_id, physics.size());
	ERR_FAIL_INDEX(p_polygon_index, physics[p_layer_id].polygons.size());
	physics.write[p_layer_id].polygons.write[p_polygon_index].one_way_margin = p_one_way_margin;
	emit_signal(SNAME("changed"));
}
float TileData::get_collision_polygon_one_way_margin(int p_layer_id, int p_polygon_index) const {
	ERR_FAIL_INDEX_V(p_layer_id, physics.size(), 0.0);
	ERR_FAIL_INDEX_V(p_polygon_index, physics[p_layer_id].polygons.size(), 0.0);
	return physics[p_layer_id].polygons[p_polygon_index].one_way_margin;
}
int TileData::get_collision_polygon_shapes_count(int p_layer_id, int p_polygon_index) const {