This PR adds a patch from upstream that fixes C++20 support. [In C++20,
template parameters from the class template are no longer allowed in
constructors and destructors
declarations](https://eel.is/c++draft/diff.cpp17#class-2). The Qhull C++
headers have a few instances of these, and don't compile under compliant
C++20 (or later). Specifically, since version 11, GCC generates an error
diagnostic.
[While this has been fixed in the
upstream](https://github.com/qhull/qhull/pull/122), the slow Qhull
release cycle means it might be quite a while longer until a new
official release it available. It's already been a year and a half since
the fix, [and the next release is still in
alpha](https://github.com/qhull/qhull/wiki#qhull-81-alpha3-20230102)
with no clear timeline. As C++20 becomes more mainstream, I believe it's
important to ensure support for this library.
