This adds the approximation of phase function convolution
of the distant lighting captured inside probe volumes.
This is based on a publication at siggraph from Bartlomiej Wronsky
"Volumetric Fog: Unified compute shader based solution to
atmospheric scattering"
Implementation is quite straightforward. However this isn't as
good as one can expect as there isn't self shadowing from the
volume themself, so the lighting is still quite flat.
To fix this, we have to add support for volumetrics inside
probe volumes baking. But this approach would still be static
so a more general solution is still to be found for dynamic
volumes like smoke simulations.
Pull Request: https://projects.blender.org/blender/blender/pulls/119479
893430a2c7