Add an option to make unlit VolumeFog invisible

[Art-Michel]

Making volumetric fog not appear when not hit by a light (example from Unity where this is the case) allows for effects such as fake Godrays that do not darken the scene

In Godot when fog is not hit by any light you get black fog that blocks the view

An option to have fog disappear when not hit by a light of a specific layer would also allow for fogs of more specific shape maybe

[Calinou]

Related to #1374.

As I described in the linked proposal, I started working on a property that allows you to use volumetric fog in a purely additive way, effectively turning it into a volumetric lighting solution. However, this needs to have per-light control on volumetric fog energy for this to be usable.

An option to have fog disappear when not hit by a light of a specific layer would also allow for fogs of more specific shape maybe

The particular example you've shown can be achieved using a FogVolume node set to a Cone shape and with an emissive color matching the light's color.