3D Gaussians splatting support

[Meakk]

3D Gaussians splatting is a new photogrammetry method used to generate a bunch of 3D Gaussians (points with more attributes like anisotropic size, and orientation)
It's very popular but there is no real consensus about the file format yet (see mkkellogg/GaussianSplats3D#47)

Let's start by adding .splat files from https://github.com/antimatter15/splat project which seem to be simple, even if it does not support spherical harmonics.
We can add more formats in the future easily as long as the architecture is ready in F3D (GPU point sort, and splat shading)