More precise mapping for mu_s in single scattering. The value for A was using the approximation  DistanceToTopAtmosphereBoundary(atmosphere, atmosphere.bottom_radius, atmosphere.mu_s_min) - d_min ~ -2.0 * atmosphere.mu_s_min * atmosphere.bottom_radius (for mu_s_min < 0). The exact value is used instead.
More precise mapping for mu_s in single scattering. The value for A was using the approximation  DistanceToTopAtmosphereBoundary(atmosphere, atmosphere.bottom_radius, atmosphere.mu_s_min) - d_min ~ -2.0 * atmosphere.mu_s_min * atmosphere.bottom_radius (for mu_s_min < 0). The exact value is used instead.

ebruneton/precomputed_atmospheric_scattering,ebruneton/precomputed_atmospheric_scattering,ebruneton/precomputed_atmospheric_scattering