### 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(), 0);
	ERR_FAIL_INDEX_V(p_polygon_index, physics[p_layer_id].polygons.size(), 0);
	return physics[p_layer_id].polygons[p_polygon_index].shapes.size();
}
Ref<ConvexPolygonShape2D> TileData::get_collision_polygon_shape(int p_layer_id, int p_polygon_index, int shape_index) const {
	ERR_FAIL_INDEX_V(p_layer_id, physics.size(), Ref<ConvexPolygonShape2D>());
	ERR_FAIL_INDEX_V(p_polygon_index, physics[p_layer_id].polygons.size(), Ref<ConvexPolygonShape2D>());
	ERR_FAIL_INDEX_V(shape_index, (int)physics[p_layer_id].polygons[p_polygon_index].shapes.size(), Ref<ConvexPolygonShape2D>());
	return physics[p_layer_id].polygons[p_polygon_index].shapes[shape_index];
}
void TileData::set_terrain_set(int p_terrain_set) {
	ERR_FAIL_COND(p_terrain_set < -1);
	if (p_terrain_set == terrain_set) {
		return;
	}
	if (tile_set) {
		ERR_FAIL_COND(p_terrain_set >= tile_set->get_terrain_sets_count());
		terrain = -1;
		for (int i = 0; i < 16; i++) {
			terrain_peering_bits[i] = -1;
		}
	}
	terrain_set = p_terrain_set;
	notify_property_list_changed();
	emit_signal(SNAME("changed"));
}
int TileData::get_terrain_set() const {