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