drawGlyphOver optimization.
drawGlyphOver optimization.

Timings (as for change 1122043) go from 49ms to 48ms ish. It's
mostly lost in the noise, but it probably doesn't hurt.

R=r
CC=golang-dev
http://codereview.appspot.com/1179041

nightlyone/golang,h8liu/golang,bdwalton/debian-golang,gnanderson/go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,nightlyone/golang,apparentlymart/golang,bdwalton/debian-golang,bdwalton/debian-golang,nightlyone/golang,ssrl/go,h8liu/golang,gnanderson/go,gnanderson/go,tianon/debian-golang,dsriram/golang,tianon/debian-golang,bdwalton/debian-golang,dsriram/golang,h8liu/golang,bdwalton/debian-golang,pombredanne/go-deleteme,apparentlymart/golang,h8liu/golang,nightlyone/golang,pombredanne/go-deleteme,tianon/debian-golang,dsriram/golang,h8liu/golang,tv42/old-go,nightlyone/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,tianon/debian-golang,tv42/old-go,h8liu/golang,tianon/debian-golang,nightlyone/golang,gnanderson/go,ssrl/go,tv42/old-go,apparentlymart/golang,tv42/old-go,h8liu/golang,ssrl/go,bdwalton/debian-golang,apparentlymart/golang,dsriram/golang,tv42/old-go,tv42/old-go,tv42/old-go,dsriram/golang,ssrl/go,dsriram/golang,bdwalton/debian-golang,gnanderson/go,nightlyone/golang,ssrl/go,tianon/debian-golang,ssrl/go,ssrl/go,tianon/debian-golang,pombredanne/go-deleteme,bdwalton/debian-golang,tianon/debian-golang