Add Rectangle examples with corner radius

samples/ControlCatalog/Pages/CanvasPage.xaml

@@ -4,7 +4,7 @@
   <StackPanel Orientation="Vertical" Spacing="4">
     <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" RadiusX="10" RadiusY="10">
         <Rectangle.OpacityMask>
           <LinearGradientBrush StartPoint="0%,0%" EndPoint="100%,100%">
             <LinearGradientBrush.GradientStops>
@@ -12,8 +12,9 @@
               <GradientStop Offset="1" Color="Transparent"/>
             </LinearGradientBrush.GradientStops>
           </LinearGradientBrush>
-        </Rectangle.OpacityMask>     
+        </Rectangle.OpacityMask>
       </Rectangle>
+      <Rectangle Fill="Green" Stroke="Black" StrokeThickness="2" Width="40" Height="20" Canvas.Left="150" Canvas.Top="10" RadiusX="10" RadiusY="5" />
       <Ellipse Fill="Green" Width="58" Height="58" Canvas.Left="88" Canvas.Top="100"/>
       <Path Fill="Orange" Data="M 0,0 c 0,0 50,0 50,-50 c 0,0 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">