replace std::codecvt with custom conversion routine from OIIO

[cary-ilm]

The deprecated std::codecvt will be entirely removed in OpenEXR v4, but until then, its use in ImfMisc.cpp generates warnings.

OIIO implemented an alternative in AcademySoftwareFoundation/OpenImageIO#5107

Bring that into ImfMisc.cpp, to eliminate the compiler warning in a way that can be patched into v3.4.

[Arcmantis18]

Hi, I hope this PR addresses the issue.

[kmilos]

Bring that into ImfMisc.cpp, to eliminate the compiler warning in a way that can be patched into v3.4.

I don't fully understand the urgency for this: although deprecated, codecvt isn't going away in 3.4 lifetime (it's not built as C++26), and the warning could as well be tolerated a little while longer. Dropping code going forward (why not for 3.5 already, soversion changes anyway, no?) is IMHO a better strategy than introducing custom code and increasing the surface for bugs and vulnerabilities.

Note that in this particular regard OpenEXR is very different to OIIO needs as there is (unfortunately) one single place there where this conversion is actually absolutely needed for all platforms (ICC string decoding), not just as a filename utility function on Windows (where native conversion is available). OIIO doesn't really have a choice. OpenEXR (fortunately) does. Treat the cause, not the symptoms.

[cary-ilm]

OK, I agree with that strategy. I'd hoped to get v3.4 building with C++26, but I agree that's not urgent.

The new plan:

1. Leave std::codecvt in place for now, warning and all. Reject Update ImfMisc.cpp for issue #2349 #2358 and Replaced std::codecvt with custom UTF-8->UTF-16 conversion code by Larry Gritz #2381.
2. Add OPENEXR_DEPRECATED around WideFilename in the public API, announcing that it will go away at some point. No code/ABI change, just a warning to OpenEXR users.
3. Add pragmas to suppress WidenFilename deprecation warnings in the internal OpenEXR test suite, to reduce the internal build noise.
4. Release that as v3.4.x. OpenEXR v3.4 will not build with C++26, but we'll live with that.
5. Remove WidenFilename (i.e. merge PR Remove WidenFilename() from public API #2314). Release that only in v4.0; don't merge to v3.4. This at least gets the OpenEXR main branch to build with C++26.