Optimize exp/draw/x11 flusher inner loop.
Optimize exp/draw/x11 flusher inner loop.

On my laptop, time to prepare and write 800x600 pixels over the
socket falls from 125-ish ms to 80-ish ms.

Thanks to Roger Peppe for the suggestion.

R=r
CC=golang-dev
http://codereview.appspot.com/1228044

firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old