Precompute per-Gaussian properties

[jatentaki]

The rendering code uses the vertex pulling technique: we have n Gaussians to draw, each corresponding to a quad, that is 2 triangles -> 6 vertices. We create a buffer gaussianData of size n, launch the vertex shader for 6n vertices and have each vertex access gaussianData[vertexIndex // 6]. This is implemented here. Since most computations are shared across all 6 vertices of a Gaussian, much of the computation (such as converting from spherical harmonics to RGB) is unnecessarily repeated 6-fold. A better implementation would be to first launch a compute shader n times, pre-compute all data shared across vertices (such as RGB) and then have the render pipeline access the precomputed values.