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.

[mvaligursky]

It's definitely on my list, both image and text source, nicely tested @querielo, loving this!
But at this stage I'm not entirely sure on the best way to implement this, as I'd want this to be generic enough to be executed per frame, possibly even a GPU version.