Added GrContext::AutoClipStack to encapsulate setting\resetting of clip stack

http://codereview.appspot.com/6343097/



git-svn-id: http://skia.googlecode.com/svn/trunk@4558 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/texdata.cpp b/gm/texdata.cpp
index 316e4d6..c3c09fb 100644
--- a/gm/texdata.cpp
+++ b/gm/texdata.cpp
@@ -89,8 +89,7 @@
                 }
                 GrAutoUnref au(texture);
 
-                GrClip newClip(GrRect::MakeWH(2*S, 2*S));
-                ctx->setClip(newClip);
+                GrContext::AutoClip acs(ctx, GrRect::MakeWH(2*S, 2*S));
 
                 ctx->setRenderTarget(target);