[mathtext] sync with upstream 2016 version

[ferdymercury]

To be applied after #23175

Supersedes #22476

Variant of #22462 but upgrading to latest version (2016) and decoupling from ROOT libs

It's basically the version of @yslai at https://github.com/yslai/mathtext/tree/master/src/libmathtext with just a few patches on top to solve compilation errors.

Future TO-DO items:

• move to LCG / Debian maybe via AddExternalProject with patch upstream some patches from ROOT and add cmake build system for libmathtext yslai/mathtext#2
• PDF output might be potentially achievable since newer version of which has fontembedpdf.cc file

[linev]

@ferdymercury

Did you check locally that all TMathText tutorials still working?
Changes are really significant in mathtext library.
Also does PNG/PDF output still works?

[ferdymercury]

Did you check locally that all TMathText tutorials still working?

Yep, I did. Looks fine. Both tutorials and their outputs.

Changes are really significant in mathtext library.
Also does PNG/PDF output still works?

PDF output never worked. Now it could work but needs changes in TPDF.cxx, so for a future PR.
PS and PNG outputs works as before.

[linev]

PDF output never worked.

Yes, you are right.

If PS and PNG output still work - then it should be fine.

[github-actions]

Test Results

    22 files      22 suites   3d 10h 1m 36s ⏱️
 3 860 tests  3 858 ✅ 0 💤 2 ❌
74 899 runs  74 896 ✅ 1 💤 2 ❌

For more details on these failures, see this check.

Results for commit d1613ed.

♻️ This comment has been updated with latest results.

[ferdymercury]

Failures seem unrelated

[ferdymercury]

(New CI failures are unrelated.)