Add GSplatProcedural class for generating GSplatData from images

[querielo]

Description

This PR adds support for procedurally generating GSplat data from images and a canvas.
It introduces a new GSplatProcedural helper that can create GSplatData instances from an image URL, with optional subsampling and per-splat radius control.

The PR includes:

• A dummy implementation to illustrate the concept.
• An example demonstrating both image-based and procedurally generated GSplat resources.
• A text-label helper that renders labels as GSplats.

output.mp4

This PR is just PoC. I think it can be helpful to have a tool in SuperSplat that allows us to create procedural splats, add markers/PoI/labels and so on.

Checklist

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

[willeastcott]

This looks super-interesting. Looking forward to getting @mvaligursky's take when he's back from vacation.