code: func (c *Context) CopyTexSubImage2D(target, level, xoffset, yoffset, x, y, w, h int) {
  	c.Call("copyTexSubImage2D", target, level, xoffset, yoffset, x, y, w, h)
  }
docs: // Replaces a portion of an existing 2D texture image with data from the current framebuffer.