Improve parity with gsplat for dense rasterization

[matthewdcong]

This set of changes improves operator parity with respect to gsplat and promotes interoperability between the frameworks.

1. Alpha threshold decreases slightly from 0.999 to 0.99
2. sigma for Gaussian influence clamping is increased slightly from 0.3 to 0.33
3. The per-axis 2d radii are both used for tile intersection as opposed to just the maximum. This reduces the number of Gaussian-tile intersections with zero contribution for anisotropic Gaussians leading to improved speed and memory savings.