Use IBitmap for Image.Source

And register converter for that interface. This is to conform with
ImageBrush.

src/Markup/Perspex.Markup.Xaml/Context/PerspexWiringContext.cs

@@ -88,7 +88,7 @@ namespace Perspex.Markup.Xaml.Context
             var typeConverterProvider = new TypeConverterProvider();
             var converters = new[]
             {
-                new TypeConverterRegistration(typeof(Bitmap), new BitmapTypeConverter()),
+                new TypeConverterRegistration(typeof(IBitmap), new BitmapTypeConverter()),
                 new TypeConverterRegistration(typeof(Brush), new BrushTypeConverter()),
                 new TypeConverterRegistration(typeof(Color), new ColorTypeConverter()),
                 new TypeConverterRegistration(typeof(Classes), new ClassesTypeConverter()),

src/Perspex.Controls/Image.cs

@@ -27,7 +27,7 @@ namespace Perspex.Controls
         /// <summary>
         /// Gets or sets the bitmap image that will be displayed.
         /// </summary>
-        public Bitmap Source
+        public IBitmap Source
         {
             get { return GetValue(SourceProperty); }
             set { SetValue(SourceProperty, value); }
@@ -48,7 +48,7 @@ namespace Perspex.Controls
         /// <param name="context">The drawing context.</param>
         public override void Render(DrawingContext context)
         {
-            Bitmap source = Source;
+            var source = Source;
 
             if (source != null)
             {