dispose method

Implementation

void dispose(){
  if(_didDispose) return;
  _didDispose = true;
  extensions.dispose();
  state.dispose();
  properties.dispose();
  capabilities.dispose();
  utils.dispose();
  info.dispose();
  _videoTextures.clear();
  _sources.clear();
  wrappingToGL.clear();
  filterToGL.clear();
}