More point splats shaders

[Meakk]

When rendering points (-o option), we are rendering a quad centered on the point and shade the fragment based on its position relative to the center

Currently, we have two shader types for rendering points:

• sphere: simulate a sphere by making the fragment darker based on the distance to the origin (default)
• gaussian: change the opacity of the fragment based on the distance to the origin using a gaussian function (used to render 3D gaussians .splat files)

We could implement more shaders like drawing a circle, a cross, or anything useful when displaying point clouds.