videotoolbox: add hwcontext support

This adds tons of code for no other benefit than making VideoToolbox
support conform with the new hwaccel API (using hw_device_ctx and
hw_frames_ctx).

Since VideoToolbox decoding does not actually require the user to
allocate frames, the new code does mostly nothing.

One benefit is that ffmpeg_videotoolbox.c can be dropped once generic
hwaccel support for ffmpeg.c is merged from Libav.

Does not consider VDA or VideoToolbox encoding.

Fun fact: the frame transfer functions are copied from vaapi, as the
mapping makes copying generic boilerplate. Mapping itself is not
exported by the VT code, because I don't know how to test.