Filter out draws to non-PMA configs that won't blend correctly

Review URL: http://codereview.appspot.com/5376059/


git-svn-id: http://skia.googlecode.com/svn/trunk@2665 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/GrGpuGL.cpp b/src/gpu/GrGpuGL.cpp
index ebf33f4..dc6ace7 100644
--- a/src/gpu/GrGpuGL.cpp
+++ b/src/gpu/GrGpuGL.cpp
@@ -1416,9 +1416,9 @@
 }
 
 void GrGpuGL::onClear(const GrIRect* rect, GrColor color) {
-    if (NULL == fCurrDrawState.fRenderTarget) {
-        return;
-    }
+    // parent class should never let us get here with no RT
+    GrAssert(NULL != fCurrDrawState.fRenderTarget);
+
     GrIRect r;
     if (NULL != rect) {
         // flushScissor expects rect to be clipped to the target.