lavu: Deprecate AVFrame.error[]

These field are difficult to interpret, and are provided by a single
encoder (mpegvideoenc). In general they do not belong to a structure
containing raw data only, so remove them from AVFrame.

Mpegvideoenc now uses a private field in Picture for its internal
computations.

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