Make constant QP mode actually have constant QP

Before this patch, recording with raspivid -t 10000 --qp NN (all default settings otherwise) the actual qp for the first 10 frames is:
--qp 20: 20, 20, 20, 22, 24, 22, 21, 20, 20, 21, 22, ...
--qp 30: 30, 30, 30, 28, 30, 28, 26, 24, 22, 20, 22, ...
and then 22-23 for most subsequent frames.  This is as seen in h264bitstream's h264_analyze output.  Also, the file sizes are the same +/- a few percent (this is the same static scene).

With this patch, reported qp is exactly as set for all frames and resulting file sizes are 35MB vs 1MB.