Add GstVaapiWindowClass::get_colormap() hook to help determine the
currently active colormap bound to the supplied window, or actually
create it if it does not already exist yet.
Add GstVaapiWindowClass::get_visual_id() function hook to help find
the best suitable visual id for the supplied window. While doing so,
also simplify the process by which an X11 window is created with a
desired Visual, i.e. now use a visual id instead of a Visual object.
Use hardware accelerated XRenderComposite() function, from the RENDER
extension, to blit a pixmap to screen. Besides, this can also support
cropping and scaling.
