feat: add fisheye projection for Gaussian splats

[mvaligursky]

Adds fisheye projection support for 3D Gaussian splatting, based on the generalized camera model from Optimal Projection for 3D Gaussian Splatting. The projection uses g(θ) = k·tan(θ/k) with a Jacobian-corrected covariance, enabling wide-angle and "little planet" effects.

Changes:

• Add GSplatParams.fisheye property (0–1 range) to control fisheye projection strength
• 0 = standard perspective (original fast path), >0 = increasing barrel distortion
• Both raster (quad) and compute rendering paths are supported
• Shader variants (with/without fisheye) are lazily compiled and cached for instant switching
• Cone-based culling replaces frustum culling when fisheye is active (compute path)
• New FisheyeProjection helper class computes all derived projection values from the normalized slider input

API Changes:

• GSplatParams.fisheye: new property (number, 0–1, default 0)

Examples:

• Updated lod-streaming example with Fisheye slider and FOV clamping for invalid states

Performance:

• Zero overhead when fisheye is disabled (original shader path, no extra uniforms)
• Fisheye shader precomputes expensive trigonometry on CPU and shares values between shader stages
• First enable has a one-off shader compilation cost; subsequent toggles are free

[Copilot]

Pull request overview

Adds fisheye projection support to the unified Gaussian splatting pipeline (quad + compute paths), including shader variants, derived projection parameter computation on CPU, and fisheye-aware culling in the compute interval pipeline.

Changes:

• Add GSplatParams.fisheye (0–1) and introduce a FisheyeProjection helper to derive projection constants (k, proj scales, maxTheta).
• Add GSPLAT_FISHEYE shader variants for both GLSL/WGSL vertex paths and WGSL compute path (projection + Jacobian-adjusted covariance).
• Switch compute interval culling to cone-based culling when fisheye is enabled; update the LOD streaming example UI with fisheye + environment controls.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/scene/shader-lib/wgsl/chunks/gsplat/vert/gsplatStructs.js	Adds fisheye per-splat shared values to the vertex center struct (WGSL).
src/scene/shader-lib/wgsl/chunks/gsplat/vert/gsplatCorner.js	Implements fisheye Jacobian path for covariance projection (WGSL).
src/scene/shader-lib/wgsl/chunks/gsplat/vert/gsplatCenter.js	Adds fisheye projection path and fisheye uniforms (WGSL).
src/scene/shader-lib/wgsl/chunks/gsplat/compute-gsplat-local-tile-count.js	Extends compute tile-count shader uniforms / call-site to support fisheye (WGSL).
src/scene/shader-lib/wgsl/chunks/gsplat/compute-gsplat-interval-cull.js	Adds fisheye cone-culling variant for interval cull (WGSL).
src/scene/shader-lib/wgsl/chunks/gsplat/compute-gsplat-common.js	Adds fisheye projection + Jacobian in shared compute covariance routine (WGSL).
src/scene/shader-lib/glsl/chunks/gsplat/vert/gsplatStructs.js	Adds fisheye per-splat shared values to the vertex center struct (GLSL).
src/scene/shader-lib/glsl/chunks/gsplat/vert/gsplatCorner.js	Implements fisheye Jacobian path for covariance projection (GLSL).
src/scene/shader-lib/glsl/chunks/gsplat/vert/gsplatCenter.js	Adds fisheye projection path and fisheye uniforms (GLSL).
src/scene/gsplat-unified/gsplat-renderer.js	Adds shared fisheyeProj helper instance to the renderer base.
src/scene/gsplat-unified/gsplat-quad-renderer.js	Toggles GSPLAT_FISHEYE define + binds fisheye uniforms for quad path.
src/scene/gsplat-unified/gsplat-params.js	Adds new GSplatParams.fisheye API property + docs.
src/scene/gsplat-unified/gsplat-manager.js	Updates culling pipeline to feed fisheye parameters and cone-culling state.
src/scene/gsplat-unified/gsplat-local-dispatch-set.js	Adds lazy caching for standard vs fisheye tile-count compute resources.
src/scene/gsplat-unified/gsplat-interval-compaction.js	Adds cached compute variants for frustum vs fisheye cone culling + parameter wiring.
src/scene/gsplat-unified/gsplat-frustum-culler.js	Stores camera cone-culling inputs (pos/forward/maxTheta) for fisheye mode.
src/scene/gsplat-unified/gsplat-compute-local-renderer.js	Selects/creates fisheye vs standard tile-count compute variant and binds uniforms.
src/scene/gsplat-unified/fisheye-projection.js	New helper computing derived fisheye projection constants from slider + camera state.
examples/src/examples/gaussian-splatting/lod-streaming.example.mjs	Adds fisheye UI wiring, FOV clamping, and HDRI environment presets/loader.
examples/src/examples/gaussian-splatting/lod-streaming.controls.mjs	Adds fisheye slider + environment selector; expands FOV slider range.

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