One-pass rate control fixes and cleanups

Fixes rate control partially in one-pass non-cbr case to achieve a
bitrate close to the one desired. Previous version was way off at
the high bitrate end.

Also includes several one-pass rate control cleanups and refactoring.

On derfraw300, one-pass encoding is now 19% off from two-pass speed
0 encoding, down from 35%.

Change-Id: I6f0dcdb7f8aa85a7e7cd3a3155d4f9d2a4d2f4f4