Add helper SkMatrix44::hasPerspective()

BUG=skia:1017
R=danakj@chromium.org, reed@google.com

Author: tomhudson@google.com

Review URL: https://codereview.chromium.org/513343002
diff --git a/include/utils/SkMatrix44.h b/include/utils/SkMatrix44.h
index 26247a0..83b5443 100644
--- a/include/utils/SkMatrix44.h
+++ b/include/utils/SkMatrix44.h
@@ -198,6 +198,10 @@
         return !(this->getType() & ~(kScale_Mask | kTranslate_Mask));
     }
 
+    inline bool hasPerspective() const {
+        return SkToBool(this->getType() & kPerspective_Mask);
+    }
+
     void setIdentity();
     inline void reset() { this->setIdentity();}