code: def ref(self):
          
          # Clean
          self._refs = [r for r in self._refs if (r() is not None)]
          # Get ref
          ref = self._refs[0]() if self._refs else None
          if ref is not None:
              return ref
          else:
              raise RuntimeError('No reference for available for GLShared')
docs: A reference (stored internally via a weakref) to an object
          that the backend system can use to obtain the low-level
          information of the "reference context". In Vispy this will
          typically be the CanvasBackend object.