pixdesc: rename PIX_FMT_* flags to AV_PIX_FMT_FLAG_*

cmdutils.c

doc/APIchanges

libavcodec/avpicture.c

libavcodec/ffv1dec.c

libavcodec/ffv1enc.c

libavcodec/imgconvert.c

libavcodec/pnm.c

libavcodec/rawdec.c

libavcodec/tiffenc.c

libavcodec/utils.c

libavcodec/xwdenc.c

libavfilter/formats.c

libavfilter/vf_crop.c

libavfilter/vf_fieldorder.c

libavfilter/vf_pixdesctest.c

libavfilter/vf_scale.c

libavutil/frame.c

libavutil/imgutils.c

libavutil/pixdesc.c

libavutil/pixdesc.h