### 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)

double picked = Math::random(0.0, sum);
	
	for (const TileMapCell &E : set) {
		if (E.source_id >= 0) {
			Ref<TileSetSource> source = sources[E.source_id];
			Ref<TileSetAtlasSource> atlas_source = source;
			if (atlas_source.is_valid()) {
				TileData *tile_data = atlas_source->get_tile_data(E.get_atlas_coords(), E.alternative_tile);
				count += tile_data->get_probability();
			} else {
				count += 1.0;
			}
		} else {
			count += 1.0;
		}
		if (count >= picked) {
			return E;
		}
	}
	ERR_FAIL_V(TileMapCell());
}
Vector<Vector2> TileSet::get_tile_shape_polygon() {
	Vector<Vector2> points;
	if (tile_shape == TileSet::TILE_SHAPE_SQUARE) {
		points.push_back(Vector2(-0.5, -0.5));
		points.push_back(Vector2(0.5, -0.5));
		points.push_back(Vector2(0.5, 0.5));
		points.push_back(Vector2(-0.5, 0.5));
	} else if (tile_shape == TileSet::TILE_SHAPE_ISOMETRIC) {
		points.push_back(Vector2(0.0, -0.5));