exp/sprite/portable: revert for /x/image/draw api

Fix broken build that was broke by "fix[ing] broken build"

Beaking change: http://golang.org/cl/13970
Reverting to state after http://golang/org/cl/12774

Change-Id: Ibb054467ca374d1f9ccff0ec1874a3e567d0c325
Reviewed-on: https://go-review.googlesource.com/16021
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

exp/sprite/portable/portable.go

@@ -187,7 +187,7 @@ func affine(dst *image.RGBA, src image.Image, srcb image.Rectangle, mask image.I
 	// TODO(nigeltao): is the caller or callee responsible for detecting
 	// transforms that are simple copies or scales, for which there are faster
 	// implementations in the xdraw package.
-	xdraw.ApproxBiLinear.Transform(dst, &m, src, srcb, xdraw.Op(op), &xdraw.Options{
+	xdraw.ApproxBiLinear.Transform(dst, m, src, srcb, xdraw.Op(op), &xdraw.Options{
 		SrcMask: mask,
 	})
 }