Add coarse energy entropy model tuning.

This tunes the entropy model for coarse energy introduced in commit
 c1c40a76.
It uses a constant set of parameters, tuned from about an hour and a
 half of randomly selected test data encoded for each frame size,
 prediction type (inter/intra), and band number.
These will be slightly sub-optimal for different frame sizes, but
 should be better than what we were using.

For inter, this saves an average of 2.8, 5.2, 7.1, and 6.7 bits/frame
 for frame sizes of 120, 240, 480, and 960, respectively.
For intra, this saves an average of 1.5, 3.0, 4.5, and 5.3 bits/frame
 (for the same frame sizes, respectively).