These were derived by analyzing the data collected for the first
iteration and applying a 3-segment piecewise-linear fit.
The boundaries were aligned to integer values of pixels-per-bit,
while optimizing for similarity to a cubic interpolation of the
control points (log-quantizer as a function of log-bitrate.)

Testing against objective-1-fast, these values appear unbiased
within a fifth of a standard deviation prior to trial encodes.
After adjusting scale by trial encode, there is no significant
bias and the standard deviation of initial frame sizes is on the
order of 3%, for all frame types except P-frames.