Remove multiplication with cos(N, lightDir) in pathtrace's explicit light sampling.

This calculation is already done in the BRDF, so in result the picture is overcompensated
for the cosine law, and is a bit too dark.
Some rearrangement in the terms of the explicit light sampling.
Add comments to pathtrace(). Change the GI test scenes a bit.
Disable stereoscopy for meshes.trinity
Add CXXPTL for multithreading.