loadTexture method
      
Future<Texture?> 
loadTexture(
    
    
- dynamic url,
- dynamic mapping,
- dynamic onLoad,
- dynamic onProgress,
- dynamic onError,
Implementation
Future<Texture?> loadTexture(url, mapping, onLoad, onProgress, onError) async {
  final manager = (this.manager != null) ? this.manager : defaultLoadingManager;
  TextureLoader? loader = manager.getHandler(url);
  if (loader == null) {
    loader = TextureLoader(manager: manager);
    loader.flipY = true;
  }
  loader.setCrossOrigin(crossOrigin);
  // final texture = loader.load( url, onLoad, onProgress, onError );
  final texture = await loader.unknown(url);
  if (mapping != null) texture?.mapping = mapping;
  return texture;
}