fixed textblock so fontstyle, font weight, etc triggers re-render.

src/Perspex.Controls/TextBlock.cs

@@ -102,6 +102,9 @@ namespace Perspex.Controls
         static TextBlock()
         {
             AffectsRender(ForegroundProperty);
+            AffectsRender(FontWeightProperty);
+            AffectsRender(FontSizeProperty);
+            AffectsRender(FontStyleProperty);
         }
 
         /// <summary>
@@ -113,7 +116,8 @@ namespace Perspex.Controls
                 this.GetObservable(TextProperty).Select(_ => Unit.Default),
                 this.GetObservable(TextAlignmentProperty).Select(_ => Unit.Default),
                 this.GetObservable(FontSizeProperty).Select(_ => Unit.Default),
-                this.GetObservable(FontStyleProperty).Select(_ => Unit.Default))
+                this.GetObservable(FontStyleProperty).Select(_ => Unit.Default),
+                this.GetObservable(FontWeightProperty).Select(_=>Unit.Default))
                 .Subscribe(_ =>
                 {
                     InvalidateFormattedText();