Stamp RGBA-alpha presence on the geometry's userData — the gsplat
member of the per-geometry presence-stamp trio
(stampPointPresenceFlags / stampLinePresenceFlags). The fixed
4-texel layout always carries the texel3.y alpha slot (1.0 identity
for RGB data), so presence is not readable off the texture;
syncGSplatMaterialWithGeometry pushes this stamp into the render
material's uHasElementAlpha gate (volumetric w(a) map) on every
commit. Refreshed on EVERY write (pool geometries are reused across
tenants) and called AFTER the texel write, matching the point/line
throw-consistency ordering (writeSplatTexels throws only at its
pre-loop guard, so old texels keep their old stamp).
Stamp RGBA-alpha presence on the geometry's userData — the gsplat member of the per-geometry presence-stamp trio (
stampPointPresenceFlags/stampLinePresenceFlags). The fixed 4-texel layout always carries the texel3.y alpha slot (1.0 identity for RGB data), so presence is not readable off the texture;syncGSplatMaterialWithGeometrypushes this stamp into the render material'suHasElementAlphagate (volumetric w(a) map) on every commit. Refreshed on EVERY write (pool geometries are reused across tenants) and called AFTER the texel write, matching the point/line throw-consistency ordering (writeSplatTexelsthrows only at its pre-loop guard, so old texels keep their old stamp).