Using render layers to enable shadow-only geometry

[rlneumiller]

RE: #18747
This fix solved one problem (preventing unwanted shadows) but from my point of view has created another (preventing wanted shadows from shadow-only geometry). A more flexible solution would allow entities to opt out of this camera-visibility filtering when they're intended to be shadow-only.

[atlv24]

there's NotShadowCaster, do we want a OnlyShadowCaster?

[rlneumiller]

That would work for my use case (think Peter Pan's mischievous shadow). I currently work around it by creating a ShadowOnlyMaterial.