aScattering2.0.shader cloud animation loops

Clouds seemed to all move out of sight after a few minutes (depending on cloudSpeed parameter values) leaving a clear sky.
This change introduces an animation loop using a general sawtooth periodic function of the form: f(t) = a * (t/a - floor(t/a)) + b:

1. start with clear sky, then
2. cloud textures pass over, then
3. back to clear sky, then
4. loop back to step 1.
Merge pull request #1 from gb96/patch-1

aScattering2.0.shader cloud animation loops