Resurrect painting.sky example as painting.dart.

I use it a lot to test and demo the painting API.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1188823008.

examples/raw/painting.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:sky' as sky;
+import 'dart:math' as math;
+import 'dart:typed_data';
+
+void beginFrame(double timeStamp) {
+  sky.PictureRecorder context = new sky.PictureRecorder(sky.view.width, 200.0);
+
+  sky.Paint paint = new sky.Paint();
+  sky.Point mid = new sky.Point(context.width / 2.0, context.height / 2.0);
+  double radius = math.min(mid.x, mid.y);
+
+  context.drawPaint(new sky.Paint()..color = const sky.Color(0xFFFFFFFF));
+
+  context.save();
+
+  context.translate(-mid.x/2.0, context.height*2.0);
+  context.clipRect(
+      new sky.Rect.fromLTRB(0.0, -context.height, context.width, radius));
+
+  context.translate(mid.x, mid.y);
+  paint.color = const sky.Color.fromARGB(128, 255, 0, 255);
+  context.rotate(math.PI/4.0);
+
+  sky.Gradient yellowBlue = new sky.Gradient.linear(
+      [new sky.Point(-radius, -radius), new sky.Point(0.0, 0.0)],
+      [const sky.Color(0xFFFFFF00), const sky.Color(0xFF0000FF)]);
+  context.drawRect(new sky.Rect.fromLTRB(-radius, -radius, radius, radius),
+                   new sky.Paint()..setShader(yellowBlue));
+
+  // Scale x and y by 0.5.
+  var scaleMatrix = new Float32List.fromList([
+      0.5, 0.0, 0.0, 0.0,
+      0.0, 0.5, 0.0, 0.0,
+      0.0, 0.0, 0.0, 0.0,
+      0.0, 0.0, 0.0, 1.0,
+  ]);
+  context.concat(scaleMatrix);
+  paint.color = const sky.Color.fromARGB(128, 0, 255, 0);
+  context.drawCircle(0.0, 0.0, radius, paint);
+
+  context.restore();
+
+  context.translate(0.0, 50.0);
+  var builder = new sky.LayerDrawLooperBuilder()
+      ..addLayerOnTop(
+          new sky.DrawLooperLayerInfo()
+            ..setOffset(const sky.Point(150.0, 0.0))
+            ..setColorMode(sky.TransferMode.src)
+            ..setPaintBits(sky.PaintBits.all),
+          (sky.Paint layerPaint) {
+        layerPaint.color = const sky.Color.fromARGB(128, 255, 255, 0);
+        layerPaint.setColorFilter(new sky.ColorFilter.mode(
+            const sky.Color.fromARGB(128, 0, 0, 255), sky.TransferMode.srcIn));
+        layerPaint.setMaskFilter(new sky.MaskFilter.blur(
+            sky.BlurStyle.normal, 3.0, highQuality: true));
+      })
+      ..addLayerOnTop(
+          new sky.DrawLooperLayerInfo()
+            ..setOffset(const sky.Point(75.0, 75.0))
+            ..setColorMode(sky.TransferMode.src)
+            ..setPaintBits(sky.PaintBits.shader),
+          (sky.Paint layerPaint) {
+        sky.Gradient redYellow = new sky.Gradient.radial(
+            new sky.Point(0.0, 0.0), radius/3.0,
+            [const sky.Color(0xFFFFFF00), const sky.Color(0xFFFF0000)],
+            null, sky.TileMode.mirror);
+        layerPaint.setShader(redYellow);
+        // Since we're don't set sky.PaintBits.maskFilter, this has no effect.
+        layerPaint.setMaskFilter(new sky.MaskFilter.blur(
+            sky.BlurStyle.normal, 50.0, highQuality: true));
+      })
+      ..addLayerOnTop(
+          new sky.DrawLooperLayerInfo()..setOffset(const sky.Point(225.0, 75.0)),
+          (sky.Paint layerPaint) {
+        // Since this layer uses a DST color mode, this has no effect.
+        layerPaint.color = const sky.Color.fromARGB(128, 255, 0, 0);
+      });
+  paint.setDrawLooper(builder.build());
+  context.drawCircle(0.0, 0.0, radius, paint);
+
+  sky.view.picture = context.endRecording();
+}
+
+void main() {
+  sky.view.setBeginFrameCallback(beginFrame);
+  sky.view.scheduleFrame();
+}