Also, modify the affine test to share the test
image with the glimage test.
LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://golang.org/cl/164400043
This code is an import from my long-defunct graphics
library, code.google.com/p/graphics-go. It was
reviewed by nigeltao back in the day.
LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://golang.org/cl/142520043