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.