* light: Add support for LIGHTING_E5BGR9.
This is not really finished. Currently mutually exclusive with
regular RGBLIGHTING, and not tested with external .lit file.
* light: replace HDR_PackResult with a version following the OpenGL spec
- add HDR_UnpackE5BRG9 for unpacking
- caller is now responsible for scaling 128 -> 1
- expose in header for tests
* light: move HDR_PackE5BRG9/HDR_UnpackE5BRG9 to common/litfile.hh
* build: std::powf not available in gcc
* common: move LoadLitFile to litfile.hh
* common: LoadLitFile: return hdr variant as well
* lightpreview: add an Exposure slider
* lightpreview: support hdr .lit's, wip
* tests: refactoring to set up testing for hdr lit's
* tests: add test for -hdr and -bspxhdr
* tests: fix LM_Sample to deal with styles correctly
* lightpreview: fix glsl error
* lightpreview: show dialog box for glsl compile errors
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Co-authored-by: Daniel Svensson <dsvensson@gmail.com>
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