This "reuses" the flags introduced for the av_vdpau_bind_context() API
function, and makes them available to all hwaccels. This does not affect
the current vdpau API, as av_vdpau_bind_context() should obviously
override the AVCodecContext.hwaccel_flags flags for the sake of
compatibility.

Cherry-picked from Libav commit 16a163b5.
Reviewed-by: Mark Thompson <sw@jkqxz.net>