Add support for SSR on web

[mate-h]

Objective

• Implemented WebGPU safe SSR (screen-space reflections) while preserving native behavior
• Fixes Atmosphere example broken on Webgpu #21700

Solution

• Kept native SSR unchanged by defining USE_DEPTH_SAMPLERS shader def on native
• the pipeline still binds filtered depth samplers and uses textureSampleLevel on depth
• added manual depth sampling in ssr/raymarch.wgsl via textureLoad with clamped bilinear and nearest helpers
• This avoids the validation error previously happening on web (documented in the linked issue above)

Testing

• Ran the ssr example on native and web using the bevy CLI
• Ran the atmosphere example on native and web

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Showcase

Atmosphere example running (fixed) using deferred rendering + SSR on web browser (chrome)