Prevent text measuring as NaN.

When text is right-aligned, WidthIncludingTrailingWhitespace seems to
return null. Worked around that but I think the layout code is wrong
now.

Windows/Perspex.Direct2D1/Media/FormattedTextImpl.cs

@@ -120,9 +120,15 @@ namespace Perspex.Direct2D1.Media
 
         public Size Measure()
         {
-            return new Size(
+            var metrics = this.TextLayout.Metrics;
-                this.TextLayout.Metrics.WidthIncludingTrailingWhitespace,
+            var width = metrics.WidthIncludingTrailingWhitespace;
-                this.TextLayout.Metrics.Height);
+
+            if (float.IsNaN(width))
+            {
+                width = metrics.Width;
+            }
+
+            return new Size(width, this.TextLayout.Metrics.Height);
         }
 
         public void SetForegroundBrush(Brush brush, int startIndex, int count)