CAVLC "trellis"

CAVLC "trellis"
~3-10% improved compression with CAVLC.
--trellis is now a valid option with CAVLC.
Perhaps more importantly, this means psy-trellis now works with CAVLC.

This isn't a real trellis; it's actually just a simplified QNS.
But it takes enough shortcuts that it's still roughly as fast as a trellis; just not quite optimal.
Thus the name is a bit of a misnomer, but we're reusing the option name because it does the same thing.
A real trellis would be better, but CAVLC is much harder to trellis than CABAC.
I'm not aware of any published polynomial-time solutions that are significantly close to optimal.