gl/glutil: fix TestImage.

The start/stop functions were introduced in
https://go-review.googlesource.com/#/c/9879/

Change-Id: I7aafadcbe78b60bb512c2de05f3f992e47f0df6e
Reviewed-on: https://go-review.googlesource.com/10372
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

gl/glutil/glimage_test.go

@@ -42,6 +42,9 @@ func TestImage(t *testing.T) {
 	ctxGL := createContext()
 	defer ctxGL.destroy()
 
+	start()
+	defer stop()
+
 	const (
 		pixW = 100
 		pixH = 100