lavc: deprecate FF_*_TYPE macros in favor of AV_PICTURE_TYPE_* enums

lavc: deprecate FF_*_TYPE macros in favor of AV_PICTURE_TYPE_* enums

Also deprecate av_get_pict_type_char() in favor of
av_get_picture_type_char().

The new enum and av_get_picture_type_char() are defined in libavutil.
This allows the use in libavfilter without the need to link against
libavcodec.

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
Signed-off-by: Anton Khirnov <anton@khirnov.net>