Collect all polygons with influence over a pixel, then process the resulting lists pixel-parallel using exact math instead of trying to abuse a few planes of Z-

Collect all polygons with influence over a pixel, then process the resulting lists pixel-parallel using exact math instead of trying to abuse a few planes of Z-buffered framebuffer to accumulate per-pixel state into a few numbers (while handling polygons virtually one-after-another, using some locking tactics of sorts to serialize their access to a single pixel's framebuffer values, if even that much concurrency-correctness is done).