exp/draw: fix drawRGBA when x0 > x1 and we have to draw right-to-left.
exp/draw: fix drawRGBA when x0 > x1 and we have to draw right-to-left.

R=r
CC=golang-dev
http://codereview.appspot.com/1940042

ssrl/go,ssrl/go,tv42/old-go,h8liu/golang,ssrl/go,h8liu/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,tv42/old-go,dsriram/golang,ssrl/go,ssrl/go,dsriram/golang,gnanderson/go,ssrl/go,tv42/old-go,gnanderson/go,h8liu/golang,dsriram/golang,tv42/old-go,ssrl/go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,apparentlymart/golang,h8liu/golang,h8liu/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,h8liu/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,gnanderson/go,apparentlymart/golang,h8liu/golang,pombredanne/go-deleteme