[FIX] Fix screen space particles when CPU simulation is used

[willeastcott]

Fixes #7534

When screenSpace: true is set on a particle system and CPU particles are used (either forced via device.supportsGpuParticles = false or when sorting is enabled), screen space particles were not rendering correctly.

Changes

• Added SCREEN_SPACE handling to the CPU particle billboard function in both GLSL and WGSL shaders
• Added SCREEN_SPACE handling to the CPU particle position output in both GLSL and WGSL shaders

Root Cause

The GPU particle shaders had proper SCREEN_SPACE conditional handling, but the CPU particle shaders were missing this logic entirely. This caused CPU particles to:

1. Use view-inverse matrix billboarding instead of simple axis-aligned billboarding for screen space
2. Transform positions through the view-projection matrix instead of outputting directly to normalized device coordinates

Testing

Tested with the user-interface/particle-system example with CPU particles forced via:

device.supportsGpuParticles = false;

Checklist

• I have read the contributing guidelines
• My code follows the project's coding standards
• This PR focuses on a single change

[Copilot]

Pull request overview

This PR fixes a bug where screen space particles were not rendering correctly when CPU simulation was used (either forced or when sorting is enabled). The fix adds the missing SCREEN_SPACE conditional handling to the CPU particle shaders, making them consistent with the GPU particle shaders.

Key Changes:

• Added SCREEN_SPACE handling to the CPU particle billboard function in both GLSL and WGSL
• Added SCREEN_SPACE handling to the CPU particle position output in both GLSL and WGSL

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/scene/shader-lib/wgsl/chunks/particle/vert/particle_cpu_end.js	Added conditional logic to output position directly to NDC for screen space particles instead of transforming through view-projection matrix
src/scene/shader-lib/wgsl/chunks/particle/vert/particle_cpu.js	Added conditional logic to use axis-aligned billboarding for screen space particles instead of view-inverse matrix billboarding
src/scene/shader-lib/glsl/chunks/particle/vert/particle_cpu_end.js	Added conditional logic to output position directly to NDC for screen space particles instead of transforming through view-projection matrix
src/scene/shader-lib/glsl/chunks/particle/vert/particle_cpu.js	Added conditional logic to use axis-aligned billboarding for screen space particles instead of view-inverse matrix billboarding

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