applying wip/gumbo-parser update soon - zathura is troubled

[Greg Troxel <gdt%lexort.com@localhost>]

I am going to apply the update wip/gumbo-parser unless there are
objections.  Kevin has tested.

The only problem is that zathura-pdf-mupdf doesn't seem to work.
However, my experience is that it fails in the same way before and
after, and therefore is not a bar to updating.

Overall, I see the new gumbo-parser as a good step.  I'll apply no
earlier than 1400Z on the 16th, absent objections with rationale.