code: func WmOpaqueRegionSet(xu *xgbutil.XUtil, win xproto.Window,
  	regions []WmOpaqueRegion) error {
  
  	raw := make([]uint, len(regions)*4)
  
  	for i, region := range regions {
  		raw[i*4+0] = uint(region.X)
  		raw[i*4+1] = uint(region.Y)
  		raw[i*4+2] = region.Width
  		raw[i*4+3] = region.Height
  	}
  
  	return xprop.ChangeProp32(xu, win, "_NET_WM_OPAQUE_REGION", "CARDINAL",
  		raw...)
  }
docs: // _NET_WM_OPAQUE_REGION set