Decoder for new PVQ code

Hasn't been tested much, but PVQ is still off by default anyway.
encode/decode: fix sizeof mismatch
Using separate contexts for expectation of PVQ gain/angle of different bands

Results in identical images, but reduces the rate by 1% at high rate to
2.5% at low rate. Measured on subset1 with forced 8x8 blocks.
Support for PVQ on 4x4 blocks.

Overall, doesn't make much difference when block switching is enabled (probably
too few 4x4 blocks?). If forcing to 4x4, it's currently a small loss at low
rate and a small gain at high rate. Probably still needs some work (bands?).