[NTG-context] different behavior between xetex and luatex

[Yue Wang]

Hi, Hans:

>
> When trk is replaced by TRK it works OK in XeTeX.

Thanks! So I think we should support lowercase code for xetex as well
for api consistency?
LuaTeX/MKIV use lowercase labels.

Then we can get similar output using the same tex file.

Yue Wang