sky: Move sky-fs to sky-preetham-fs
sky: Fix variable name
scene: Use glm::perspective instead of glm::perspectiveFov
Add simple sky model (single color) for performance comparisons
sky: Implement Hosek-Wilkie model

The code uses spline data set 1.4a from the sample implementation
sky: Fix singularity at theta=pi/2 in Hosek-Wilkie model
sky: Fixes and refactoring

Perez formula in Preetham model uses the stabilizing fudge factor modification from Hosek-Wilkie