Make quads rectangular (currently square)

[jatentaki]

The current vertex shader computes the radius = max(gaussian_sigma_x, gaussian_sigma_y) and then draws square quads of size (radius, radius). It is up to the fragment shader to actually draw them as thin/wide, depending on the vertex data. Drawing the quads in shape (sigma_x, sigma_y) would be significantly more efficient for non-square splats (saves a lot of fragment shader invocations) but requires readjusting some vertex shader code as well.