svq3: fix decoding residual blocks of b-frames.

The residual block data of 16x16 blocks was ignored for b-frames, which
leads to easy-to-identify artifacts. After this patch, the artifacts are
gone. Sample video: svq3_watermark.mov. (Fate results unaffected.)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>