Make GetAnchor abstract

There are subtle reasons for this. Regardless, it will be abstract when it's an interface, anyway, so no harm in doing it now.

Terminal.Gui/View/Layout/Dim.cs

@@ -233,7 +233,7 @@ public abstract record Dim : IEqualityOperators<Dim, Dim, bool>
     ///     subclass of Dim that is used. For example, DimAbsolute returns a fixed dimension, DimFactor returns a
     ///     dimension that is a certain percentage of the super view's size, and so on.
     /// </returns>
-    internal virtual int GetAnchor (int size) { return 0; }
+    internal abstract int GetAnchor (int size);
 
     /// <summary>
     ///     Calculates and returns the dimension of a <see cref="View"/> object. It takes into account the location of the

Terminal.Gui/View/Layout/DimAuto.cs

@@ -59,6 +59,9 @@ public record DimAuto : Dim
     /// <inheritdoc/>
     public override string ToString () { return $"Auto({Style},{MinimumContentDim},{MaximumContentDim})"; }
 
+    /// <inheritdoc />
+    internal override int GetAnchor (int size) => 0;
+
     internal override int Calculate (int location, int superviewContentSize, View us, Dimension dimension)
     {
         var textSize = 0;