code: func GetPortion(bit C.MMBitmapRef, x, y, w, h int) C.MMBitmapRef {
  	var rect C.MMRect
  	rect.origin.x = C.size_t(x)
  	rect.origin.y = C.size_t(y)
  	rect.size.width = C.size_t(w)
  	rect.size.height = C.size_t(h)
  
  	pos := C.get_portion(bit, rect)
  	return pos
  }
docs: // GetPortion get bitmap portion