Adds the new av_hwframe_map() function, which allows mapping between
hardware frames and normal memory, along with internal support for
implementing it.
Also adds av_hwframe_ctx_create_derived(), for creating a hardware
frames context associated with one device using frames mapped from
another by some hardware-specific means.
The source frame may be cropped, so that its dimensions are smaller than
the pool dimensions. The transfer_data API requires the allocated size
of the destination frame to be the same as the pool size.
