AO was overwriting decals because they don't use SSOverlay, and they would then overwrite AO overlays because they don't have a plane/layer.
This adds those, so decals will now correctly render and AO will render on top of them (and various other things).
Various changes to accidental whitespace etc., plane updated to ABOVE_TURF_PLANE and AO_LAYER (12). Hiding mobs and shallow liquids will now appear above AO.
AO was overwriting decals because they don't use SSOverlay, and they would then overwrite AO overlays because they don't have a plane/layer. This adds those, so decals will now correctly render and AO will render on top of them (and various other things).
Before - No decals next to wall where there's AO:
After - AO on top of decals.