Function quantizeColor3

Quantises a single RGB colour from normalised [0..1] floats into the [0..255] byte range, returning the result in a caller-supplied dest (allocates if omitted).

This is the conversion the WebGL renderer applies before writing into its RGBA8UI per-view attribute texture. Centralising it here keeps the convention in one place — both the initial-upload path (viewing!webGLRenderer.internal.GPUMemoryBatch | GPUMemoryBatch) and the colour-change / colorize-clear paths (viewing!webGLRenderer.internal.RendererMesh | RendererMesh) agree on it.

Differences from compressRGBColors:

• Operates on a single Vec3 rather than a flat [r,g,b, r,g,b, …] array.
• Writes into dest instead of allocating a fresh typed array — safe to call from the hot render path.
• Uses Math.floor, matching the original renderer convention (compressRGBColors rounds, which would disagree at 0.5-boundary values).
• Does not throw on out-of-range input — the bottom byte simply wraps. Callers who care should clamp upstream.