Skybox blocks shadows

[Saratii]

I also noticed that the directional light is hardcoded in the plugin. That is too invasive, doesn't allow proper configuration, and would interfere with other scene lights.

I have a PR ready that fixes both of these issues and adds another example, once my other is merged.
https://github.com/Saratii/bevy-volumetric-clouds/tree/shadows_being_blocked