Added stack to AA clip mask cache

http://codereview.appspot.com/6201058/



git-svn-id: http://skia.googlecode.com/svn/trunk@3885 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/GrSoftwarePathRenderer.cpp b/src/gpu/GrSoftwarePathRenderer.cpp
index cbec965..22e4a26 100644
--- a/src/gpu/GrSoftwarePathRenderer.cpp
+++ b/src/gpu/GrSoftwarePathRenderer.cpp
@@ -232,6 +232,7 @@
             // rendering (kGlyphMaskStage in GrBatchedTextContext)
             kPathMaskStage = GrPaint::kTotalStages,
         };
+        GrAssert(NULL == target->drawState()->getTexture(kPathMaskStage));
         target->drawState()->setTexture(kPathMaskStage, texture);
         target->drawState()->sampler(kPathMaskStage)->reset();
         GrScalar w = GrIntToScalar(pathBounds.width());