Temporal blur of the estimated frame complexity, before curve
compression (default: 20). Lower values allow the quantizer
value to jump around more, higher values force it to vary more
smoothly. cplx_blur ensures that each I-frame has quality com-
parable to the following P-frames, and ensures that alternating
high and low complexity frames (e.g. low fps animation) do not
waste bits on fluctuating quantizer.
