Re-implement the PSSM_SPLITS debug option

[clayjohn]

This uses a render_mode instead of shadow_color to avoid adding a cost to the basic shader

I am not sure this is the optimal way to implement this, but I am uploading for now to help debug work in #76291

We should discuss if render_mode is the right way to implement this. Right now it is the only feasible option that doesn't require either:

1. exposing light splits to user shaders
2. a new way of adding single defines into shaders

But we may want to implement an API for creating debug modes of shaders

A simple alternative that I have seen is to run a full screen pass that reads from the depth buffer and tints everything based on distance. This wouldn't be limited just to shadows, but it would outline the cascades just as effectively and be an even simpler, more self-contained implementation

[BastiaanOlij]

It's a shame this doesn't work for custom materials where we change the vertex position and it would be nice if we think of a solution for this, but just what this adds right now is already a big improvement.

[akien-mga]

Thanks!