code: func (ml MetalLayer) NextDrawable() (MetalDrawable, error) {
  	md := C.MetalLayer_NextDrawable(ml.metalLayer)
  	if md == nil {
  		return MetalDrawable{}, errors.New("nextDrawable returned nil")
  	}
  
  	return MetalDrawable{md}, nil
  }
docs: // NextDrawable returns a Metal drawable.
  //
  // Reference: https://developer.apple.com/documentation/quartzcore/cametallayer/1478172-nextdrawable.