public static class ShadowUtil.OccludersExtractor
OccludersExtractor is a helper class to collect splitOccluders from scene recursively.
It utilizes the scene hierarchy, instead of making the huge flat geometries list first.
Instead of adding all geometries from scene to the RenderQueue.shadowCast and checking
all of them one by one against camera frustum the whole Node is checked first
to hopefully avoid the check on its children.