experimental speedup some xfermodes with Sk4f

bench/XfermodeBench.cpp

Index: bench/XfermodeBench.cpp
diff --git a/bench/XfermodeBench.cpp b/bench/XfermodeBench.cpp
index c5cb714104aac7cfff07d91a48ce1cc71e6b7ed1..1dddd8743052ad66cd33700b84565da171be9971 100644
--- a/bench/XfermodeBench.cpp
+++ b/bench/XfermodeBench.cpp
@@ -46,7 +46,9 @@ protected:
                 w,
                 h
             );
-            canvas->drawRect(rect, paint);
+            for (int j = 0; j < 1000; ++j) {
+                canvas->drawRect(rect, paint);
+            }
         }
     }