Remove IMutablePen.

It's not really needed. Mutable pens will always be `Pen`s.

src/Avalonia.Visuals/Media/BrushExtensions.cs

@@ -50,7 +50,7 @@ namespace Avalonia.Media
         {
             Contract.Requires<ArgumentNullException>(pen != null);
 
-            return pen as ImmutablePen ?? ((IMutablePen)pen).ToImmutable();
+            return pen as ImmutablePen ?? ((Pen)pen).ToImmutable();
         }
     }
 }

src/Avalonia.Visuals/Media/IMutablePen.cs

@@ -1,17 +0,0 @@
-using System;
-using Avalonia.Media.Immutable;
-
-namespace Avalonia.Media
-{
-    /// <summary>
-    /// Represents a mutable pen which can return an immutable clone of itself.
-    /// </summary>
-    public interface IMutablePen : IPen, IAffectsRender
-    {
-        /// <summary>
-        /// Creates an immutable clone of the pen.
-        /// </summary>
-        /// <returns>The immutable clone.</returns>
-        ImmutablePen ToImmutable();
-    }
-}

src/Avalonia.Visuals/Media/Pen.cs

@@ -11,7 +11,7 @@ namespace Avalonia.Media
     /// <summary>
     /// Describes how a stroke is drawn.
     /// </summary>
-    public class Pen : AvaloniaObject, IMutablePen, IEquatable<IPen>
+    public class Pen : AvaloniaObject, IPen, IEquatable<IPen>
     {
         /// <summary>
         /// Defines the <see cref="Brush"/> property.