Use the flags argument of av_hwframe_ctx_create_derived() to pass the
mapping flags which will be used on allocation. Also, set the format
and hardware context on the allocated frame automatically - the user
should not be required to do this themselves.
Some frames contexts are not usable without additional format-specific
state in hwctx. This change adds new functions frames_derive_from and
frames_derive_to to initialise this state appropriately when deriving
a frames context which will require it to be set.
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.