Fix weather particle fading

Closes #5455 (closed).

Also fix weird rain material setup (based on mp3butcher's PR).

I've added a particle operator to all weather particle system modular programs that does what AlphaFaders still do for non-particle geometry for particle systems. The current weather particle alpha is passed through the use of a reference to mEffectFade variable saved into the sky manager class, which might be controversial.

This doesn't fix anything about the lighting or the fact that shaders aren't used for weather particles due to them being a part of the sky.

Some messy rain intensity uniform value handling has been cleaned up (result intensity used for rain ripples will be slightly lower).