T8x8 zbin and rate control changes.

T8x8 zbin and rate control changes.

Some adjustments to zbin for t8x8.
Changes to rules for sizing forced key frames.
Some extra stats output in tmp.stt.

Approximate  gain on YT-hd set 0.5%

There are still issues in sizing key frames and gf/arf frames
when the image is largely static. These in part relate to
problems with cost estimates in the recode loop.

Change-Id: I6f0159dc8a8faeab4115a19c668d442491619a68