Fix: support RenderLayer filtering for lights

[pauliusuza]

Objective

• Fixes Regression: RenderLayers do not work for Lights #23264
• Adds an example in /examples/3d/light_render_layers.rs

Prior art:

• light renderlayers #10742

Solution

• Added per-light packed render-layer masks (stored in unused high bits of existing light flags) and used them during shading.
• Added a packed render-layer mask to mesh uniforms/input uniforms, propagated through GPU preprocess and meshlet resolve.
• Forward PBR lighting now filters point/spot/directional lights by mesh_layers ∩ light_layers at fragment time.
• Shadow specialization/queue now also filters by light-vs-mesh layers (not just camera-vs-mesh), so wrong-layer meshes no longer cast into a light’s shadow pass.

With this implementation, per-mesh/per-light filtering supports layers 0..=25 (26 layers total).

• These 26 bits are packed into light flags starting at bit 6.
• If a mesh/light uses layers outside that packed range, it falls back to permissive behavior for those cases (preserves previous behavior instead of breaking rendering).

Testing

Tested and confirmed working with:
cargo run --example light_render_layers

---

[github-actions]

Welcome, new contributor!

Please make sure you've read our contributing guide and we look forward to reviewing your pull request shortly ✨

[pcwalton]

Again, all of this will collide with #23107

[pcwalton]

This will collide with #23107. It's not a blocker though.

[pcwalton]

I totally get why you wanted to save memory here, but I think I'd just add a field instead. As you wrote it, we have no room to add flags; if we wanted to add flags we'd have to add a field anyway or risk breaking apps. And 26 is kind of an arbitrary number; 32 would be less weird.

[pauliusuza]

The light flags field is a u32, and the lower 6 bits are already used for light feature flags (bit 0..bit 5), so render-layer bits must start at shift 6, leaving exactly 26 bits for layer membership.