add isRect() check to AAClip, to detect if a soft-clip is really just an irect

src/core/SkRasterClip.cpp

Index: src/core/SkRasterClip.cpp
diff --git a/src/core/SkRasterClip.cpp b/src/core/SkRasterClip.cpp
index 664211f64f3e2a504bd85ff569c9afeb2045565e..d1615a3445f6348fef5e3cf7cdd3d3a916fa8aa0 100644
--- a/src/core/SkRasterClip.cpp
+++ b/src/core/SkRasterClip.cpp
@@ -222,7 +222,10 @@ void SkRasterClip::convertToAA() {
     SkASSERT(fIsBW);
     fAA.setRegion(fBW);
     fIsBW = false;
-    (void)this->updateCacheAndReturnNonEmpty();
+    
+    // since we are being explicitly asked to convert-to-aa, we pass false so we don't "optimize"
+    // ourselves back to BW.
+    (void)this->updateCacheAndReturnNonEmpty(false);
 }
 
 #ifdef SK_DEBUG