code: func (document *Document) PaintTile(buf unsafe.Pointer, canvasWidth int, canvasHeight int, tilePosX int, tilePosY int, tileWidth int, tileHeight int) {
  	C.paint_tile(
  		document.handle,
  		(*C.uchar)(buf),
  		(C.int)(canvasWidth),
  		(C.int)(canvasHeight),
  		(C.int)(tilePosX),
  		(C.int)(tilePosY),
  		(C.int)(tileWidth),
  		(C.int)(tileHeight),
  	)
  }
docs: // PaintTile renders tile to given buf (which size must be a `4 * canvasWidth * canvasHeight`).
  // In practice buf must be a pointer to image.Image.Pix array's first element, e.g. unsafe.Pointer(&image.Pix[0])