tidbits: use C++ cmath functions

[tritao]

Summary

• Use std::isinf and std::isnan from <cmath> in tidbits.cpp.
• Preserve coin_isinf()'s documented -1 / +1 return contract with std::signbit.

Motivation

FreeCAD snap builds exposed a portability issue with GCC 13/C++20: Coin detects isinf and isnan through the C math.h header, but tidbits.cpp is C++ code including <cmath>, where the portable names are in the std:: namespace. Some environments do not expose the unqualified C names from <cmath>.

[tritao]

@kadet1090 Want to review and merge this one?

[kadet1090]

I'm fine with the changes but not sure if we want to merge it before upstream had a change to look at it.

[tritao]

This change actually fixes behavior here to match the documented function return value, which the previous version did not.

But upon an audit, this is not even used, at least on Linux, so there's not much risk of this correctness fix causing an issue.

[tritao]

I'm fine with the changes but not sure if we want to merge it before upstream had a change to look at it.

We need it to fix the snap build in FreeCAD, so I think we should merge it for now, especially since it's quite a basic fix.

If upstream wants changes to be done, we can just re-bump the submodule with those fixes.

But opened PR against upstream: coin3d#638

[kadet1090]

Fine with me, won't be too much problem anyway.