Factor code to rotate a canvas about a point.

gm/strokes.cpp

Index: gm/strokes.cpp
diff --git a/gm/strokes.cpp b/gm/strokes.cpp
index 61f5a8af6aa770895e4c135c5ac7b04f4f3b3e64..39661a5a07c8533157ff9e3da7489f67c505db1d 100644
--- a/gm/strokes.cpp
+++ b/gm/strokes.cpp
@@ -10,6 +10,7 @@
 #include "SkRandom.h"
 #include "SkDashPathEffect.h"
 #include "SkParsePath.h"
+#include "sk_tool_utils.h"
 
 #define W   400
 #define H   400
@@ -290,12 +291,6 @@ protected:
         return SkISize::Make(W, H*2);
     }
 
-    static void rotate(SkScalar angle, SkScalar px, SkScalar py, SkCanvas* canvas) {
-        SkMatrix matrix;
-        matrix.setRotate(angle, px, py);
-        canvas->concat(matrix);

[bungeman-skia, 2016/07/12 15:47:11]

With floats the order of operations matters. This code will produce slightly
different numbers from the new code.

-    }
-
     void onDraw(SkCanvas* canvas) override {
         canvas->drawColor(SK_ColorWHITE);
 
@@ -316,7 +311,7 @@ protected:
             for (int i = 0; i < N/2; i++) {
                 SkRect r;
                 rnd_rect(&r, &paint, rand);
-                rotate(SkIntToScalar(15), SW/2, SH/2, canvas);
+                sk_tool_utils::rotate_about(canvas, SkIntToScalar(15), SW/2, SH/2);
                 canvas->drawPath(fPath, paint);
             }
         }