video: support filtering hardware frames via libavfilter

Requires a bunch of hacks:
- we access AVFilterLink.hw_frames_ctx. This is not a public API in
  FFmpeg and Libav. Newer FFmpeg provides an accessor
  (av_buffersink_get_hw_frames_ctx), but it's not available in Libav or
  the current FFmpeg release or Libav. We need this value after filter
  graph creation, so We have no choice but to access this.
  One alternative is making filter creation and format negotiation
  fully lazy (i.e. delay it and do it as filters are output), but this
  would be a huge change.
  So for now, we knowingly violate FFmpeg's and Libav's ABI and API
  constraints because they don't provide anything better.
  On newer FFmpeg, we use the (quite ugly) accessor, though.
- mp_image_params doesn't (and can't) have a field for the frames
  context AVBufferRef. So we pass it via vf_set_proto_frame(), and even
  more hacks.
- if a filter needs a hw context, but we haven't created one yet
  (because normally we create them lazily), it will fail at init.
- we allow any hw format now, although this could go horrible wrong.

Why all this effort? We could move hw deinterlacing filters etc. to
FFmpeg, which is a very worthy goal.

player/video.c

@@ -701,6 +701,7 @@ static int video_decode_and_filter(struct MPContext *mpctx)
 
     if (vo_c->input_mpi) {
         vo_c->input_format = vo_c->input_mpi->params;
+        vf_set_proto_frame(vo_c->vf, vo_c->input_mpi);
 
         if (vo_c->is_coverart && !vo_c->cached_coverart)
             vo_c->cached_coverart = mp_image_new_ref(vo_c->input_mpi);