[C] Invalidate Grid when row/col def is replaced (#13320) fixes #13154

We invalidate when the items of the row/col def are modified. With this,
we also invalidate when the full collection is swapped.

- fixes #13154

Xamarin.Forms.Core/Grid.cs

@@ -30,6 +30,7 @@ public partial class Grid : Layout<View>, IGridController, IElementConfiguration
 					((ColumnDefinitionCollection)oldvalue).ItemSizeChanged -= ((Grid)bindable).OnDefinitionChanged;
 				if (newvalue != null)
 					((ColumnDefinitionCollection)newvalue).ItemSizeChanged += ((Grid)bindable).OnDefinitionChanged;
+				((Grid)bindable).OnDefinitionChanged(bindable, EventArgs.Empty);
 			}, defaultValueCreator: bindable =>
 			{
 				var colDef = new ColumnDefinitionCollection();
@@ -44,6 +45,7 @@ public partial class Grid : Layout<View>, IGridController, IElementConfiguration
 					((RowDefinitionCollection)oldvalue).ItemSizeChanged -= ((Grid)bindable).OnDefinitionChanged;
 				if (newvalue != null)
 					((RowDefinitionCollection)newvalue).ItemSizeChanged += ((Grid)bindable).OnDefinitionChanged;
+				((Grid)bindable).OnDefinitionChanged(bindable, EventArgs.Empty);
 			}, defaultValueCreator: bindable =>
 			{
 				var rowDef = new RowDefinitionCollection();