vf_hwmap: Add reverse mapping for hardware frames

This is something of a hack.  It allocates a new hwframe context for
the target format, then maps it back to the source link and overwrites
the input link hw_frames_ctx so that the previous filter will receive
the frames we want from ff_get_video_buffer().  It may fail if
the previous filter imposes any additional constraints on the frames
it wants to use as output.

(cherry picked from commit 81a4cb8e)