Fix Dozeu X-drop a different way

[adamnovak]

Changelog Entry

To be copied to the draft changelog by merger:

• dozeu respects X-drop definition more strictly

Description

This takes some fixes that @jeizenga and @mgawan hammered out that fix a bug in traceback in Dozeu, using @mgawan's code.

This goes with vgteam/dozeu#17.

I tried to also merge in vgteam/dozeu#16, but I ended up with a Dozeu commit with, as far as I could tell, no non-comment code changes versus vgteam/dozeu#17 but which made vg fail the vg test "[xdrop]" tests. So this is taking @mgawan's version as merged into Dozeu. @mgawan said some things I didn't really understand about the changes not actually working on our mainline Dozeu, which differed from what they were built on by what looks to me like a no-op merge commit, so if something goes wrong here we might need to do some serious Git surgery to unravel what's going on.

This closes #4997, a competing PR to do the same basic thing

[adamnovak]

I don't want to merge this until I'm sure that @mgawan is happy with that Dozeu commit actually going into the project. If not, we'd want to back it out again.

[mgawan]

All is good from our end. Thanks for working on it.

I believe I had missed Jordan's more recent commits on his PR. But looks like we both ended up with the same end fix.