code: func WmOpaqueRegionGet(xu *xgbutil.XUtil,
  	win xproto.Window) ([]WmOpaqueRegion, error) {
  
  	raw, err := xprop.PropValNums(xprop.GetProperty(xu, win,
  		"_NET_WM_OPAQUE_REGION"))
  	if err != nil {
  		return nil, err
  	}
  
  	regions := make([]WmOpaqueRegion, len(raw)/4)
  	for i, _ := range regions {
  		regions[i] = WmOpaqueRegion{
  			X:      int(raw[i*4+0]),
  			Y:      int(raw[i*4+1]),
  			Width:  raw[i*4+2],
  			Height: raw[i*4+3],
  		}
  	}
  	return regions, nil
  }
docs: // _NET_WM_OPAQUE_REGION get