This selects the quantizer to use for P-frames. I- and B-frames
are offset from this value by ip_factor and pb_factor, respec-
tively. 20-40 is a useful range. Lower values result in better
fidelity, but higher bitrates. 0 is lossless. Note that quan-
tization in H.264 works differently from MPEG-1/2/4: H.264's
quantization parameter (QP) is on a logarithmic scale. The map-
ping is approximately H264QP = 12 + 6*log2(MPEGQP). For exam-
ple, MPEG at QP=2 is equivalent to H.264 at QP=18.
