Factor code to rotate a canvas about a point.

src/core/SkCanvas.cpp

Index: src/core/SkCanvas.cpp
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index 0ce97fff20661f8e2c82aacb801d2bb1cf574718..3067772c8ad87db06b7ac588f14c61afd141239a 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -1498,6 +1498,12 @@ void SkCanvas::rotate(SkScalar degrees) {
     this->concat(m);
 }
 
+void SkCanvas::rotate(SkScalar degrees, SkScalar px, SkScalar py) {

[reed1, 2016/07/12 17:28:59]

possibly check for 0 == degrees to skip the rest. Just a thought.

[bungeman-skia, 2016/07/12 17:37:42]

If so, I think it would need to be done above as well. If you think it would be
common enough to be interesting, we can put together a CL to do that.

It turns out that this is already better than the translate-rotate-translate
calls from Patch Set 1, since the matrix is being produced more efficiently and
there is only one concat (as opposed to three concats).

+    SkMatrix m;
+    m.setRotate(degrees, px, py);
+    this->concat(m);
+}
+
 void SkCanvas::skew(SkScalar sx, SkScalar sy) {
     SkMatrix m;
     m.setSkew(sx, sy);