Use shadowCasterPS chunk for gsplat shadow output

[mvaligursky]

Follow-up to #9116 — migrates the unified gsplat shadow caster to the shadowCasterPS chunk instead of a hardcoded output.

Changes:

• chunks/gsplat/frag/gsplat.js (GLSL + WGSL) now #include "shadowCasterPS" under SHADOW_PASS and write getShadowOutput(), replacing vec4(gl_FragCoord.z, 0, 0, 1). The gsplat shadow material is rendered through the standard shadow pass, so it already receives the LIGHT_TYPE/SHADOW_TYPE/PERSPECTIVE_DEPTH defines from Add shadow pass support for ShaderMaterial using #include #9116 — no extra plumbing needed. Gsplat shadows are directional-only, which is fully within the chunk's supported range.
• This fixes VSM shadows for gsplats: the previous hardcoded output wrote raw depth with a zero coverage value, which the EVSM receiver treats as "not covered", making VSM shadows effectively not work for gsplats. PCSS output is unchanged; PCF is unaffected (the shadow pass forces color-write-off and uses hardware depth for that path).

Examples:

• gaussian-splatting/shadows gained a Shadow Type dropdown (all types, applied to all lights), plus vsmBlurSize/penumbraSize/penumbraFalloff tuned so VSM and PCSS look correct out of the box.

[github-actions]

Build size report

This PR changes the size of the minified bundles.

Bundle	Minified	Gzip	Brotli
playcanvas.min.js	2326.5 KB (+0.1 KB, +0.00%)	598.5 KB —	465.3 KB (+0.2 KB, +0.05%)
playcanvas.min.mjs	2323.9 KB (+0.1 KB, +0.00%)	597.4 KB (+0.0 KB, +0.00%)	464.6 KB (+0.2 KB, +0.04%)