Add support for linear gradients, implemented as skia shaders.

sky/examples/raw/painting.sky

 <sky>
 <style>
 div {
     height: 200px;
     background-color: lightblue;
 }
 </style>
 <div id="canvas" />
 <script>
 import 'dart:math' as math;
 import 'dart:typed_data';
 import 'dart:sky';
 
 void main() {
   var element = document.getElementById('canvas');
   element.requestPaint((PaintingContext context) {
     Paint paint = new Paint();
     Point mid = new Point(context.width / 2.0, context.height / 2.0);
     double radius = math.min(mid.x, mid.y);
 
     context.save();
 
     context.clipRect(new Rect.fromLTRB(0.0, 0.0, context.width, radius));
 
     context.translate(mid.x, mid.y);
     paint.color = const Color.fromARGB(128, 255, 0, 255);
     context.rotateDegrees(45.0);
 
+    Gradient yellowBlue = new Gradient.Linear(
+        [new Point(-radius, -radius), new Point(0.0, 0.0)],
+        [const Color(0xFFFFFF00), const Color(0xFF0000FF)],
+        null);
     context.drawRect(new Rect.fromLTRB(-radius, -radius, radius, radius),
-                     paint);
+                     new 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 Color.fromARGB(128, 0, 255, 0);
@@ skipping 30 matching lines @@
           layerPaint.color = const Color.fromARGB(128, 255, 0, 0);
         });
     paint.setDrawLooper(builder.build());
     context.drawCircle(0.0, 0.0, radius, paint);
 
     context.commit();
   });
 }
 </script>
 </sky>