lavc: deprecate CODEC_FLAG_INPUT_PRESERVED

The addition of reference-counted frames makes it pointless.
lavc: move CODEC_FLAG_NORMALIZE_AQP to mpegvideo
lavc: make the xvid-specific "gmc" flag a private option of libxvid
lavc: move CODEC_FLAG_MV0 to mpegvideo
mov: export stsd Compressorname in metadata

Stop using the undocumented to-be-deprecated AVCodecContext.codec_name
field.
oggparseskeleton: do not use AVCodecContext.codec_name

That field is undocumented, of dubious use, and will be deprecated.
asfenc: use codec descriptors instead of AVCodecs to write codec info

Also, stop using AVCodecContext.codec_name as fallback, since it will be
deprecated.

Changes the result of the lavf-asf test (and its associated seektest),
since 'msmpeg4v3' gets written instead of just 'msmpeg4'.
rmdec: do not export anything to AVCodecContext.codec_name

That field will be deprecated and the value that is written there is not
particularly useful.
lavc: do not use AVCodecContext.codec_name in avcodec_string()

That field will be deprecated.
lavc: deprecate AVCodecContext.codec_name

It is undocumented and has no real use.