lavc: Deprecate AVPicture structure and related functions

This structure served as a bridge between data pointers and frames,
but it suffers from several limitations:
- it is not refcounted and data must be copied to every time
- it cannot be expanded without ABI break due to being used on the stack
- its functions are just wrappers to imgutils which add a layer of
  unneeded indirection, and maintenance burden
- it allows hacks like embedding uncompressed data in packets
- its use is often confusing to our users

AVFrame provides a much better API, and, if a full blown frame is not
needed, it is just as simple and more straightfoward to use data and
linesize arrays directly.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>