Upstream issue affecting Arch packages

Description:

libxml2 2.13.2 introduced a bug affecting Arch packages. Affected packages are mathjax, and as a result of that; sigil. Any browser that relies on the system libxml2 (Chromium, QtWebengine, etc...) can be affected, really. Prevents mathjax from rendering some mathml in such browsers.

There's been about four critical fix commits to libxml2 2.13.2 (including the one I'm reporting). I'll leave it up to others to determine if cherry-picks are warranted.

Edited by Doug