[dvipdfmx] forward references

[Sašo Živanović]

Dear dvipdfmx maintainers,

I am developing an externalization package "memoize", currently
available at https://github.com/sasozivanovic/memoize.

The package is currently tested on various langsci books, and we have
hit a problem with externalizing shadings,
https://github.com/langsci/25/issues/140.

I have tracked down the problem to what I would naively call the
resolution of "forward references" in xdvipdfmx. "Naively", because I
have no idea how that program works, and I'm not even sure whether to
call the problem a feature or a bug. The issue is explained in detail in
https://github.com/sasozivanovic/memoize/issues/1.

As the above implies, the problem is limited to XeTeX; the
externalization of shadings works fine in pdfTeX and LuaTeX.

I would appreciate it if you could take the time to see if the second
approach to the resolution of the issue (as proposed in the github
issue) is feasible. And if it is, I would of course be very happy if the
issue could be addressed.

Best,
Sašo Živanović