Merge commit '1d12a545ce828eaf4fb37295400008ea37635ab8'

* commit '1d12a545ce828eaf4fb37295400008ea37635ab8':
  cbs: Add an explicit type for coded bitstream unit types

Mostly already present from 6734eef6b8b464139fdc140ec9bc9e8d74173869, two
minor message changes are merged.

Merged-by: Mark Thompson <sw@jkqxz.net>

diff --cc libavcodec/cbs_mpeg2.c
Simple merge