Revert "Prefix PixelFormat and PIX_FMT_ with AV"

This reverts commit 0a1ae660a0056e98af5da195285c81e00ba07c24.

Let's not... yet.
mp_image: extend refcounting API
vd_lavc: refactor decoder image retrieval
vd_lavc: more some more code around
video: make use of libavcodec refcounting

Now lavc_dr1.c is not used anymore if libavcodec is recent enough.
af_lavcac3enc: use audioconvert.h instead of channel_layout.h