Add an API to set the ColorFilter on a Paint object.

sky/examples/raw/painting.sky

Index: sky/examples/raw/painting.sky
diff --git a/sky/examples/raw/painting.sky b/sky/examples/raw/painting.sky
index e295abd3c6f784ca8c117c5d82492a90a7f43162..9e6d0301bbd3cf7ef458425504351d1226724cba 100644
--- a/sky/examples/raw/painting.sky
+++ b/sky/examples/raw/painting.sky
@@ -40,18 +40,25 @@ void main() {
 
     context.restore();
 
+    context.translate(0.0, 50.0);
     var builder = new LayerDrawLooperBuilder()
         ..addLayerOnTop(
-            new DrawLooperLayerInfo()..setOffset(150.0, 0.0)..setPaintBits(-1),
+            new DrawLooperLayerInfo()
+              ..setOffset(150.0, 0.0)..setPaintBits(-1)..setColorMode(1),
             (Paint layerPaint) {
-          // TODO(mpcomplete): This won't do anything until we add support for
-          // setting the color filter.
-          layerPaint.setARGB(128, 0, 0, 255);
+          layerPaint.setARGB(128, 255, 255, 0);
+          layerPaint.setColorFilter(new ColorFilter(0x770000ff, 5));
         })
         ..addLayerOnTop(
-            new DrawLooperLayerInfo()..setOffset(75.0, 75.0)..setPaintBits(0),
+            new DrawLooperLayerInfo()..setOffset(75.0, 75.0)..setColorMode(1),
             (Paint layerPaint) {
           layerPaint.setARGB(128, 255, 0, 0);
+        })
+        ..addLayerOnTop(
+            new DrawLooperLayerInfo()..setOffset(225.0, 75.0),
+            (Paint layerPaint) {
+          // Since this layer uses a DST color mode, this has no effect.
+          layerPaint.setARGB(128, 255, 0, 0);
         });
     paint.setDrawLooper(builder.build());
     context.drawCircle(0.0, 0.0, radius, paint);