Add blurcircles2 GM

gm/SkAnimTimer.h

Index: gm/SkAnimTimer.h
diff --git a/gm/SkAnimTimer.h b/gm/SkAnimTimer.h
index 2c8a723e5bb970d0799e1a802536ad5e6715b95a..2456eed4f835949a6f249c0bac8b0c2ee6587b65 100644
--- a/gm/SkAnimTimer.h
+++ b/gm/SkAnimTimer.h
@@ -104,6 +104,23 @@ public:
         return SkDoubleToScalar(value);
     }
 
+    /**
+     * Transitions from ends->mid->ends linearly over period seconds. The phase specifies a phase
+     * shift in seconds.
+     */

[robertphillips, 2016/05/13 18:44:16]

Isn't this more commonly known as ping-pong?

[bsalomon, 2016/05/13 18:58:10]

Done.

+    SkScalar pulse(SkScalar period, SkScalar phase, SkScalar ends, SkScalar mid) const {
+        return Pulse(this->secs(), period, phase, ends, mid);
+    }
+
+    /** Helper for computing a pulse value without a SkAnimTimer object */
+    static SkScalar Pulse(SkScalar t, SkScalar period, SkScalar phase, SkScalar ends,
+                          SkScalar mid) {
+        double value = ::fmod(t + phase, period);
+        double half = period / 2.0;
+        double diff = ::fabs(value - half);
+        return ends + (1.0 - diff / half) * (mid - ends);
+    }
+
 private:
     double  fBaseTimeNanos;
     double  fCurrTimeNanos;