Make sure to initialize one GstVaapiDisplay at a time, even in threaded
environments. This makes sure the display cache is also consistent
during the whole display creation process. In the former implementation,
there were risks that display cache got updated in another thread.
Make sure that libgstvaapi private headers remain internally used to
build libgstvaapi libraries only. All header dependencies were reviewed
and checks for IN_LIBGSTVAAPI definition were added accordingly.
Also rename GST_VAAPI_CORE definition to IN_LIBGSTVAAPI_CORE to keep
consistency.
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.).
