OpenEXRCore

This directory includes the sources for the OpenEXRCore 3.2.0 library, and a unity build file, openexr-c.c that compiles the sources into the Hio library. The sources are concatenated into a unity build in order that all symbols are available within a single compilation unit. All functions are adorned with either an EXR_EXPORT macro, or an EXR_INTERNAL macro, both of which are set to static here to prevent symbols from leaking out of Hio. In this way, the interred OpenEXRCore library will not conflict with any user-preferred OpenEXR libraries that may be on the system and otherwise linking to an application or plugin.

A few changes are still in progress to upstreamed to the OpenEXR project, but these are minor, and otherwise, almost all differences between the interred OpenEXRCore and the official OpenEXR repo are consolidated to the openexr_conf.h header. The remaining differences are removing an extern marking from the few global data tables.

With regards to the maintainability of the unity concatenation, the OpenEXR team has indicated a desire to have the concatenated build available as part of the standard code distribution. As such, any maintenance with respect to that file will be handled by CI on the EXR project itself. A unit test will be landed upstream to ensure this is reliably in place, however the exact approach is still in discussion.