Optimize shader binding changes.

This change also cleans up the internal API a little bit by using mat4
everywhere instead of float[16] (for the ortho matrix for instance.)

Change-Id: I35924c7dc17bad17f30307118d5ed437c2ed37e0
diff --git a/libs/hwui/Snapshot.h b/libs/hwui/Snapshot.h
index 17ca440..ef9269f 100644
--- a/libs/hwui/Snapshot.h
+++ b/libs/hwui/Snapshot.h
@@ -130,7 +130,7 @@
     /**
      * Contains the previous ortho matrix.
      */
-    float orthoMatrix[16];
+    mat4 orthoMatrix;
 
 private:
     // Clipping rectangle mapped with the transform