Anisotropic gaussians
This MR introduces support for anisotropic Gaussians (or 3D Gaussians).
Instead of having a single scalar value to describe the scale of the Gaussian resulting on an isotropic Gaussian (ie spherical shape),
it's now possible to specify a 3D scale and a rotation resulting in anisotropic Gaussian (ie ellipsoid shape).
This MR also changes the logic of the splat creation by using the EWA splatting method which should be more precise than just flipping the quad to face the camera.
A "low-pass matrix" can also be specified to apply a convolution on the computed Gaussian covariance matrix. It is used in the 3D Gaussian splatting paper and actually very important to correctly visualize very thin Gaussians. The default value is zero, so no convolution is applied.