Occlusion Culling

| Occlusion culling
| Pixie supports occlusion culling during the scan-line rendering. This is accomplished by rendering the scene front-to-back. The renderer will not shade the grids that are known to be occluded. The previous versions of Pixie (before 1.4.1) accomplished this using by keeping track of the furthest opaque fragment within a bucket. Newer versions of Pixie implement a much more robust algorithm.