dispose method
      
void
dispose()
      
     
    
    
Implementation
void dispose(){
  if(_didDispose) return;
  _didDispose = true;
  renderer.dispose();
  cubemaps.dispose();
  extensions.dispose();
  capabilities.dispose();
  bindingStates.dispose();
  clipping.dispose();
  _customShaders.dispose();
  programs.clear();
}