Set TextFormattingMode for GlyphRun

Is there any reason why `TextFormattingMode` is fixed to `Ideal` for `GlyphRun`s?

I need it to be `Display` and the only current way to do that is setting `_textFormattingMode` through reflection.

I suggest adding a public property that is settable only during initialization like all the others:

```C#
public TextFormattingMode TextFormattingMode
{
get
{
CheckInitialized();
return _textFormattingMode;
}
set
{
CheckInitializing(); // This can only be set during initialization.
_textFormattingMode = value;
}
}
```

Another option would be making `GlyphRun.TryCreate` method public.

https://github.com/dotnet/wpf/blob/master/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/GlyphRun.cs