correctly setting size to cope with text wrapping.

src/Skia/Avalonia.Skia/FormattedTextImpl.cs

@@ -590,7 +590,7 @@ namespace Avalonia.Skia
             else
             {
                 var lastLine = _skiaLines[_skiaLines.Count - 1];
-                _size = new Size(lastLine.Width, lastLine.Height);
+                _size = new Size(maxX, lastLine.Top + lastLine.Height);
             }
         }