Fix NaNs in scattering integral

[mate-h]

Objective

• Fix NaNs in scattering integral
• Fix when the sky turns dark in case of 1 mie scatterer or no scatterers

Solution

• Add max(extinction, MIN_EXTINCTION) to the scattering integrand denominator to avoid division by zero.
• The value vec3(1e-12) chosen is arbitrary and I found it has no visual impact on existing coefficients. Can be lowered by a few orders of magnitude safely though.

Testing

• Current atmosphere example and my own branch with alterations to the atmosphere example