SSGIExample: Add Cornell Box-Inspired Scene

[zalo]

Related issue: #31839

Description
Switch the current scene out for a Cornell Box inspired scene to better show off the SSGI.

Live Demo

I also considered the canonical Cornell Box, but after trying it, I felt it was too cramped.

I can't actually bake new screenshots on my machine due to some jimp error...

[Copilot]

Pull Request Overview

This PR replaces the existing Mirror's Edge apartment scene with a Cornell Box-inspired scene to better showcase SSGI (Screen Space Global Illumination) post-processing effects.

Key changes:

• Removed GLTF model loading and replaced it with procedurally generated geometry
• Created a Cornell Box scene with colored walls, boxes, and proper lighting setup
• Updated camera positioning and controls to fit the new scene layout

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[Mugen87]

I can't actually bake new screenshots on my machine due to some jimp error...

I'll update after the merge.