Imath includes shall be guarded

[sanguinariojoe]

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You are many times doing dangerous includes:

https://github.com/AcademySoftwareFoundation/openexr/blob/main/src/lib/OpenEXRCore/internal_coding.h#L24

While you should do the guarded version:

#include <Imath/half.h>

There are 2 main reasons that make your way suboptimal:

• Collisions. How many projects can have a half.h header file? Any other project with such a file, which is either not guarding the header or including it like you do, will cause a collision
• Exotic makefiles. With CMake this is not a big deal (or maybe it is...), but it is definitely a pain in the ass on other building systems. It is indeed impossible to use Imath when installed on a custom folder.

[meshula]

That would be a good thing to address, thanks for pointing it out.