Expose compose shader customization along with the example

[mvaligursky]

Summary

• Adds a minimal, extensible pattern to customize the compose shader using include chunks at runtime (no engine source edits required).
• Provides a working example that applies a circular-dot pixelation post-effect on top of the composed result, with UI controls
• Ensures WebGL (GLSL) and WebGPU (WGSL) parity.
• these are the new empty chunks the user can override (along with any existing)

        composeDeclarationsPS
        composeMainStartPS
        composeMainEndPS