glamor-egl: Added new function to create image for a pixmap.
This function is used to support dri2. In the underlying driver, it will create a buffer object for a given pixmap. And then call this api to create a egl image from that buffer object, and then bind that image to a texture, and then bind that texture to the pixmap. Normally, this pixmap's content is shared between a dri2 client and the x server. Signed-off-by: Zhigang Gong <email@example.com>