Add gsplat shader rings example using the gsplatModifyPS hook

[mvaligursky]

Adds a minimal example demonstrating the gsplatModifyPS fragment chunk added in #8872, and fixes a WGSL uniformity issue that prevented user chunks from using derivatives.

Changes:

• The WGSL gsplat fragment shader no longer early-returns after its discards in the forward path. The conditional returns made control flow non-uniform, causing WebGPU validation errors for any user gsplatModifyPS chunk calling derivative builtins (fwidth, dpdx, ...). WGSL discard demotes the invocation to a helper, so dropping the returns preserves semantics while keeping control flow uniform.

Examples:

• New gaussian-splatting/shader-rings example: each splat of the skull scene is rendered as a sharp ring of its own color. The fragment chunk uses gaussianUV for the ring shape, fwidth to keep the ring a constant screen-space pixel width at any zoom, and a screen-position-phased sin curve so a highlight wave travels across the rings. Ring width (in pixels) and ring alpha are exposed as sliders, shaders live in separate .frag files, and spherical harmonics colors update every degree of camera movement.