Don't expose GstVaapiContext APIs and make them totally private to
libgstvaapi core library. That API would also tend to disappear in
a future revision. Likewise, don't expose GstVaapiDisplayCache API
but keep symbols visible so that the various render backends could
share a common display cache implementation in libgstvaapi.
Try to clean-up the documentation from any stale entry too.
Make it possible to add extra an extra filter to most of display cache
lookup functions so that the GstVaapiDisplay instance can really match
a compatible and existing display by type, instead of relying on extra
string tags (e.g. "X11:" prefix, etc.).
