Change it from explicit metadata about every hwaccel method to trying to
get it from libavcodec. As shown by add_all_hwdec_methods(), this is a
quite bumpy road, and a bit worse than expected.
This will probably cause a bunch of regressions. In particular I didn't
check all the strange decoder wrappers, which all cause some sort of
special cases each. You're volunteering for beta testing by using this
commit.
One interesting thing is that we completely get rid of mp_hwdec_ctx in
vd_lavc.c, and that HWDEC_* mostly goes away (some filters still use it,
and the VO hwdec interops still have a lot of code to set it up, so it's
not going away completely for now).
eb8957cea1