Sort avcodec mapping table by AVCodecID enum

This makes it much easier to tell which codec ID mappings are missing
(or intentionally skipped).

Comments have been added for each codec ID that is currently missing.

Ordering between codecs with the same avcodec ID or VLC fourcc has been
preserved.

No functional change.
Signed-off-by: Rafaël Carré <funman@videolan.org>