Add gst_vaapi_surface_new_from_buffer_proxy() helper function to
create a VA surface from an external buffer provided throug the
new GstVaapiBufferProxy object.
Make it possible to create VA surfaces with a specific pixel format.
This is a new capability brought in by VA-API >= 0.34.0. If that
capability is not built-in (e.g. using VA-API < 0.34.0), then
gst_vaapi_surface_new_with_format() will return NULL.
Add helper macros to retrieve the VA surface information like size
(width, height) or chroma type. This is a micro-optimization to avoid
useless function calls and NULL pointer re-checks in internal routines.
gst_vaapi_surface_get_parent_context() was not meant to be exposed globally.
It's just an internal helper function. However, it's still possible to get
the parent context through the "parent-context" property.
