OpenEXR / IlmBase / Imath Cmake config needs overhaul

[hobbes1069]

Since OpenEXR 3.0 IlmBase is were included in OpenEXR and the library has been renamed, as of 3.0.x it has now been split off into a separate library Imath (yes very annoying).

FIeld3D should be updated to use the CMake Targets and then fallback to manual methods if it doesn't work. I've started the process but I honestly don't have time to complete it.

Does Field3D actually need OpenEXR or just Imath? From what I can tell updating the headers it looks like just Imath.

[hobbes1069]

BTW, here's the porting guide:

https://github.com/AcademySoftwareFoundation/Imath/blob/master/docs/PortingGuide2-3.md

[hobbes1069]

And my patch for Fedora

https://src.fedoraproject.org/rpms/Field3D/blob/rawhide/f/Field3D-openexr.patch