LightUp v6

LightUp v5.9c

Lots of fixes and goodness in this release!
- GPU raytracing of IES light sources
- Fixes for GPU raytraced area emitters
- Fixes for sRGB render mode (try it out LightUp->Graphics->Advanced->Use Linear sRGB)
- Fixes for SU2018+ occasionally showing graphics corruption
- Faster physical sky rendering
- Fix for Roughness overflow
- Fix for showing SketchUp fog in TourTool