code: func (im *Image) XExpPaint(wid xproto.Window, x, y int) {
  	if im.Pixmap == 0 {
  		return
  	}
  	xproto.CopyArea(im.X.Conn(),
  		xproto.Drawable(im.Pixmap), xproto.Drawable(wid), im.X.GC(),
  		int16(im.Rect.Min.X), int16(im.Rect.Min.Y),
  		int16(x), int16(y),
  		uint16(im.Rect.Dx()), uint16(im.Rect.Dy()))
  }
docs: // XExpPaint achieves a similar result as XPaint and XSurfaceSet, but
  // uses CopyArea instead of setting a background pixmap and using ClearArea.
  // CreatePixmap must be called before using XExpPaint.
  // XExpPaint can be called on sub-images.
  // x and y correspond to the destination x and y to copy the image to.
  //
  // This should not be used on the same image with XSurfaceSet and XPaint.