[XeTeX] Checking for existence of font feature

[Arthur Reutenauer]

> \font\a="Times New Roman:+smcp" at 12pt
>
> doesn't register as causing any problems, even though this call doesn't  
> produce anything different to
>
> \font\a="Times New Roman" at 12pt

  This doesn't solve your problems, but generally speaking, enabling a
feature that's not present in the font should not be expected to cause
any problem: it will only have no effect, that's all.

	Arthur