LightUp v2.6e

This release has focussed on improving workflow as well as improving stability.

- Much faster and stable handling large models
- Much faster starting/stopping with lots of Scenes
- Material Presets (lightup.materials)
- More IOR presets
- Kcutoff coefficient slider control
- Improved reflections
- 'Turn off Area Emitters' checkbox
- RTcontrols (F5 to hide)