Make gst_vaapi_display_get_display_type() return the actual VA display
type. Conversely, add a gst_vaapi_display_get_class_type() function to
return the type of the GstVaapiDisplay instance. The former is used to
identify the display server onto which the application is running, and
the latter to identify the original object class.
Use hardware accelerated XRenderComposite() function, from the RENDER
extension, to blit a pixmap to screen. Besides, this can also support
cropping and scaling.
Implement the new render-to-pixmap API. The only supported pixmap format
that will work is xRGB, with native byte ordering. Others might work but
they were not tested.
