Hosek-Wilkie (2012) analytic sky model.

Implements the Hosek & Wilkie sky luminance model as presented in “An Analytic Model for Full Spectral Sky-Dome Radiance” SIGGRAPH 2012. Compared to the older Preetham model, Hosek-Wilkie provides better accuracy at low sun elevations and more realistic sunset colouring.

The additional Albedo parameter controls ground albedo (reflected light contribution).