Introduce gst_vaapi_surface_proxy_new_from_pool() to allocate a new surface
proxy from the context surface pool. This change also makes sure to retain
the parent surface pool in the proxy.
Besides, it was also totally useless to attach/detach parent context to
VA surface each time we acquire/release it. Since the whole context owns
all associated VA surfaces, we can mark this as such only once and for all.
Make it possible to specify the maximum number of references to use within
a single VA context. This helps reducing GPU memory allocations to the useful
number of references to be used.
