Add C++ based support for drawAtlas

This is supposed to make Viktor's game faster, but it's not clear
to me that it actually does.  I've left the code
using the dart version of drawAtlas for now until Viktor can
verify that it looks correct.

I also added a wrapper for SkFilterQuality in the process of
debugging SkCanvas.drawAtlas since all drawAtlas examples
in Skia use FilterQuality.low.  The bug which blocked me for
so long turned out to be that SkCanvas.drawAtlas doesn't
draw anything if antialiasing is turned on.

Issue #138.

R=abarth@google.com

packages/flutter/example/game/lib/particle_system.dart

@@ -272,16 +272,22 @@ class ParticleSystem extends Node {
       colors.add(particleColor);
     }
 
-    drawAtlas(canvas, texture.image, transforms, rects, colors, TransferMode.modulate,
-              new Paint()..setTransferMode(transferMode));
+    // TODO(viktork): Verify that the C++ version looks right and is faster!
+    Paint paint = new Paint()..setTransferMode(transferMode)
+        ..setFilterQuality(FilterQuality.low) // All Skia examples do this.
+        ..isAntiAlias = false; // Antialiasing breaks SkCanvas.drawAtlas?
+    // return canvas.drawAtlas(texture.image, transforms, rects, colors,
+    //   TransferMode.modulate, null, paint);
+
+    dartDrawAtlas(canvas, texture.image, transforms, rects, colors,
+      TransferMode.modulate, paint);
   }
 
   double randMinus1To1() => _rand.nextDouble() * 2.0 - 1.0;
 }
 
-// TODO: Needs bindings to Skia method in SkCanvas (exclude canvas parameter)
-void drawAtlas(Canvas canvas, Image image, List<RSTransform> transforms, List<Rect> rects, List<Color> colors,
-               TransferMode transferMode, Paint paint) {
+void dartDrawAtlas(Canvas canvas, Image image, List<RSTransform> transforms,
+    List<Rect> rects, List<Color> colors, TransferMode transferMode, Paint paint) {
   assert(transforms.length == rects.length && transforms.length == colors.length);
 
   Texture mainTexture = new Texture(image);
@@ -308,13 +314,3 @@ void drawAtlas(Canvas canvas, Image image, List<RSTransform> transforms, List<Re
     canvas.restore();
   }
 }
-
-// TODO: Needs bindings to Skia SkRSXform
-class RSTransform {
-  double scos;
-  double ssin;
-  double tx;
-  double ty;
-
-  RSTransform(this.scos, this.ssin, this.tx, this.ty);
-}