Factor code to rotate a canvas about a point.

gm/imagefilterstransformed.cpp

Index: gm/imagefilterstransformed.cpp
diff --git a/gm/imagefilterstransformed.cpp b/gm/imagefilterstransformed.cpp
index 7ceff001351e1cba1944c72313d2d2b2b953c7cb..af1d16bd3b2a6c412e983cd0128c02625ed8b254 100644
--- a/gm/imagefilterstransformed.cpp
+++ b/gm/imagefilterstransformed.cpp
@@ -140,18 +140,14 @@ DEF_SIMPLE_GM(rotate_imagefilter, canvas, 500, 500) {
 
         canvas->translate(150, 0);
         canvas->save();
-            canvas->translate(100, 100);
-            canvas->rotate(30);
-            canvas->translate(-100, -100);
+            sk_tool_utils::rotate_about(canvas, 30, 100, 100);
             canvas->drawRect(r, paint);
         canvas->restore();
 
         paint.setAntiAlias(true);
         canvas->translate(150, 0);
         canvas->save();
-            canvas->translate(100, 100);
-            canvas->rotate(30);
-            canvas->translate(-100, -100);
+            sk_tool_utils::rotate_about(canvas, 30, 100, 100);
             canvas->drawRect(r, paint);
         canvas->restore();