Added explicit cache clearing

http://codereview.appspot.com/6492094/



git-svn-id: http://skia.googlecode.com/svn/trunk@5444 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/GrRenderTarget.cpp b/src/gpu/GrRenderTarget.cpp
index 7619664..f8f9e3c 100644
--- a/src/gpu/GrRenderTarget.cpp
+++ b/src/gpu/GrRenderTarget.cpp
@@ -101,6 +101,10 @@
             context->unlockStencilBuffer(fStencilBuffer);
         }
         fStencilBuffer->unref();
+
+        if (NULL != context) {
+            context->purgeCache();
+        }
     }
 
     fStencilBuffer = stencilBuffer;