sky: Render a sky sphere with fixed color
sky: First cut of Preetham implementation

The code runs but produces an incorrect result :(
sky: Fix zenith chromacity, add Reinhard tone mapping

The sky is not correct but is closer
sky: Fixed computation issues with Preetham model

It seems that the results are now correct; the model is of moderate quality however.
Since environment is LDR and sky is HDR, we resort to using Reinhard tone-mapping in sky shader for now.
ui: Fix sliders with non-zero minimum value
sky: Replace Reinhard tone mapping with normalized sun luminance

This is convenient for low-dynamic range since we can have a brightened sun (thus faking a sun disk)
Slightly reduce the sun disk size