Preetham (1999) analytic sky model.
Implements the Preetham et al. sky luminance model via Perez functions (zenithY/x/y + five Perez coefficients per channel). Physically attenuated solar spectrum is computed via Rayleigh scattering, aerosol attenuation, and ozone / water-vapor absorption.
This shader is added twice, once for backwards compatibility (“SunSky”) and once with the proper scientific name (“Preetham”)