[gl] avoid crash with system libftgl

[ferdymercury]

Fixes #22076

(Maybe) since ulrichard/ftgl@84869ec#diff-1462936246cba8b5e9ee1c79a1207cea8efb0658be58cd25e7d3acd817ac0ac0R374

so 2.1.3, there is a regression that does not correctly find symbols above code 191. So clamp the text to that max to avoid crashes, if we are using system-ftgl which is for most distributions 2.4 and for some older ones 2.1.3.

Do not clamp for builtin-ftgl since it was forked at 2.1.2, before the regression happened

More details in the linked issue.

fyi @osschar

All latex*.C scripts now run without crashing!

[github-actions]

Test Results

    22 files      22 suites   3d 10h 45m 59s ⏱️
 3 850 tests  3 849 ✅ 0 💤 1 ❌
76 948 runs  76 947 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 49c6c9b.

[linev]

PR goes in right direction, but there should be a way to use all symbols with external ftgl.
We knew that integral symbol fails. So we need to understand which glyph is used with built-in ftgl version.
And then try to find symbol which will give us same glyph with external ftgl.

And most probably such re-coding will be simple - like symb += 0x800.

[ferdymercury]

And most probably such re-coding will be simple - like symb += 0x800.

What do you think about ulrichard/ftgl@84869ec#diff-1462936246cba8b5e9ee1c79a1207cea8efb0658be58cd25e7d3acd817ac0ac0R374

Isn't the fact that it assumes UTF8 multibytes preventing us from doing such simple remapping that you propose?
The missing symbols (int epsilon etc) are exactly the ones they define as multibytes in their table. So I do not know how to fix that without modifying FTGL itself?

[linev]

I am off until Monday, but then I will try to investigate this.
As I mentioned - I still did not understand difference between builtin and external ftgl.

[ferdymercury]

I still did not understand difference between builtin and external ftgl

Builtin (2.1.2) converts each char to one glyph in order.

External (2.1.3) converts each char to one glyph in order if char <192. If char between 192 and 192+16, then it reads also the next char at once and then does the conversion. If 192+32 it's three bytes it assumes.

That's why, when I just prevented nullptr access, I was seeing Int being converted to three random glyphs. So each char int, nabla or epsilon was getting two or three glyphs together, not just one. Probably accessing garbage memory.

[linev]

External (2.1.3) converts each char to one glyph in order if char <192. If char between 192 and 192+16, then it reads also the next char at once and then does the conversion. If 192+32 it's three bytes it assumes.

But then solution is clear - one need to transform string so that final result lead to same set of glyph.