vl: improve vlc functions and handling

vl: improve vlc functions and handling

Only initialize vlc in MPEG2 decoding once for all slices,
add more sanity checks to vlc decoding functions, support
multiple vlc input buffer, improve documentation of the
vlc functions.

v2: also implement multiple inputs for the vlc functions
v3: some bug fixes for buffer size and alignment corner cases
v4: rework of the patch, some more improvements

Signed-off-by: Maarten Lankhorst <m.b.lankhorst@gmail.com>
Signed-off-by: Christian König <deathsimple@vodafone.de>