avcodec/mpeg4video: Make tables used to initialize VLCs smaller

avcodec/mpeg4video: Make tables used to initialize VLCs smaller

Switching from ff_init_vlc_sparse() to ff_init_vlc_from_lengths()
allows to replace codes which are so long that they need to be stored
in an uint16_t by symbols which fit into an uint8_t; and even these can
be avoided in case of the sprite trajectory VLC.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>