avcodec/vp3: Use symbols table for VP3 motion vectors

avcodec/vp3: Use symbols table for VP3 motion vectors

Expressions like array[get_vlc2()] can be optimized by using a symbols
table if the array is always the same for a given VLC. This requirement
is fulfilled for the VLC used for VP3 motion vectors. The reason it
hasn't been done before is probably that the array in this case
contained entries in the range -31..31; but this is no problem with
ff_init_vlc_from_lengths(): Just apply an offset of 31 to the symbols
before storing them in the table used to initialize VP3 motion vectors
and apply an offset of -31 when initializing the actual VLC.

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