Add support for loading exr files for color grading LUTs

[JonnyOThan]

Color grading supports loading an external LUT file, but it needs to be in an HDR format to be useful. This seems promising: https://github.com/takazerker/EXRImporter

Note that some kind of hdr/exr loader could probably be useful to a lot of different mods and might make more sense in KSPCF or some other small utility mod.