avcodec/msmpeg4dec: Reuse identical MV VLC

avcodec/msmpeg4dec: Reuse identical MV VLC

msmpeg4dec and ituh263dec both create VLCs with identical parameters out
of ff_mvtab. Given that ff_msmpeg4_decode_init() always (indirectly) calls
ff_h263_decode_init_vlc(), the VLC initialized by the latter can be
directly used by msmpeg4dec. Doing so saves a bit more than 2KB from the
.bss segment as well as the code to initialize a VLC.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>