Add helper functions to describe GstVaapiPoint and GstVaapiRectangle
structures as a standard GType. This could be useful to have them
described as a GValue later on.
A rendering mode can be "overlay" or "texture"'ed blit.
The former mode implies that a VA surface used for rendering can't be
re-used right away for decoding, so the sink shall make provisions to
retain the associated surface proxy until the next surface is to be
displayed.
The latter mode implies that the VA surface is implicitly copied to an
intermediate backing store, or back buffer of a frame buffer, so the
associated surface proxy can be disposed right away.
