Transferred all multiple scattering into GI-stage, improved scattering and also some optimizations done.
The sun is located behind the camera at the right side. ac3 - 34 minutes of rendering time, ac4 - 30 minutes.
ac4 - the same as ac3, but camera pitch angle is much less, so it is easy to see that GI-based scattering works bad at the distance due to low resolution and GI blurring. Also there are some need-to-fix bugs in air shadowing by clouds resulting in fake shadows at the lower side of clouds layer. This is due to some optimizations.
I'll try to implement "hybrid" algorithm to improve distant clouds view, performing the part of calculations at pixel-based rendering stage.