rate: Refine scale/exp values with training data
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.