ShaderMaterial: support omni light and spot VSM/PCSS shadow casting

Follow-up to #6835 and #9116.

The shadowCasterPS chunk added in #9116 lets a custom shader (ShaderMaterial) cast shadows via #include "shadowCasterPS" and getShadowOutput(). The initial implementation covers:

• Directional lights — all shadow types (PCF, VSM, PCSS)
• Spot lights — PCF only

Not yet supported:

• Omni lights — no shadow types
• Spot lights — VSM and PCSS

Why these were skipped

These cases don't use rasterized perspective depth. They need distance-based depth, min(distance(view_position, vPositionW) / light_radius, 0.99999), which requires:

• a world-space position varying supplied by the user's vertex shader (there is no fixed naming convention to rely on for custom shaders),
• the view_position and light_radius uniforms,
• for the non-VSM/PCSS path, writing the computed depth to gl_FragDepth.

The gl_FragDepth requirement is the main API design problem: in WGSL, output.fragDepth lives on the main function's FragmentOutput struct and cannot be written from a chunk helper function, so the clean no-argument getShadowOutput() API used for the perspective-depth cases does not extend to it symmetrically. Spot PCSS additionally needs linearized depth.

Possible approaches

• A statement-style include used inside main() (like litUserMainEndPS is used in litShadowMain), which writes gl_FragColor/gl_FragDepth (GLSL) or output.color/output.fragDepth (WGSL), relying on the engine convention that the WGSL output variable is named output.
• Or an explicit getShadowDepth(worldPos) helper that returns the depth, which the user assigns to output.fragDepth / gl_FragDepth themselves, gated behind a chunk-provided define.

Either way the user's vertex shader needs to provide the world position.

See the existing lit implementation for reference: src/scene/shader-lib/glsl/chunks/lit/frag/pass-shadow/litShadowMain.js (and the WGSL equivalent).