This supports retrieving the device from a provided hw_frames_ctx, and
automatically creating a hw_frames_ctx if hw_device_ctx is set.

The old API is not deprecated yet. The user can still use
av_vdpau_bind_context() (with or without setting hw_frames_ctx), or use
the API before that by allocating and setting hwaccel_context manually.

Cherry-picked from Libav commit 1a7ddba5.
(Adds missing APIchanges entry to the Libav version.)
Reviewed-by: Mark Thompson <sw@jkqxz.net>