Added OpacityMask example to ControlCatalog.

samples/ControlCatalog/Pages/CanvasPage.xaml

@@ -3,7 +3,15 @@
     <TextBlock Classes="h1">Canvas</TextBlock>
     <TextBlock Classes="h2">A panel which lays out its children by explicit coordinates</TextBlock>
     <Canvas Background="Yellow" Width="300" Height="400">
-      <Rectangle Fill="Blue" Width="63" Height="41" Canvas.Left="40" Canvas.Top="31"/>
+      <Rectangle Fill="Blue" Width="63" Height="41" Canvas.Left="40" Canvas.Top="31">
+        <Rectangle.OpacityMask>
+          <LinearGradientBrush StartPoint="0%,0%" EndPoint="100%,100%">
+            <LinearGradientBrush.GradientStops>
+              <GradientStop Offset="0" Color="Black"/>
+              <GradientStop Offset="1" Color="Transparent"/>
+            </LinearGradientBrush.GradientStops>
+          </LinearGradientBrush>
+        </Rectangle.OpacityMask>      </Rectangle>
       <Ellipse Fill="Green" Width="58" Height="58" Canvas.Left="88" Canvas.Top="100"/>
       <Path Fill="Orange" Data="M 0,0 c 50,0 50,-50 c 50,0 50,50 h -50 v 50 l -50,-50 Z" Canvas.Left="30" Canvas.Top="250"/>
       <Path Fill="OrangeRed" Canvas.Left="180" Canvas.Top="250">