Shadow layers

[Lielay9]

Related: #3606, #4443

To not fall into the XY-trap, I'll start off with a description of the problem I'm facing.

Currently, I'm struggling to control the shadows in my project to achieve the look I want. Essentially, I'm trying to eliminate certain objects from casting shadows on themselves but still receive shadows and cast shadows on others. A quick search on the internet tells me that I'm not the first to face such problems. Many of the solutions, however, aren't that suited for real-time applications, especially for mobile. However, if we can accept some limitations, shadow layers might be a passable solution.

Proposal in 6 sentences: Shadow Layers

Description

Add layers to lights or geometry-instances to choose which visual layers they cast their shadows on.

Solves:

1. Shadow caster doesn't have to be the same shape as the receiver.*
2. Materials can react differently to shadows cast from lights on different layers.

*Achievable by having shadow mesh on another layer than the visual. Limitation: cannot receive shadows from others with the same layer setup (acceptable in my project).

Implementation

Create a shadow map per layer the light is on.

[Lielay9]

Idk how to close these, but I created a better proposal for something that better meets my needs/ addresses the problem at #5841