Implicit Tiling: Consolidate redundancies in availability calculations

[nithinp7]

A quick glance at QuadtreeAvailability.cpp and OctreeAvailability.cpp will show a bunch of redundant code that could be extracted out given some untangling.

@javagl started to do this as mentioned in this comment: https://github.com/CesiumGS/cesium-native/pull/420#issuecomment-1018622521

We should continue his work there, doing something similar in OctreeAvailability.cpp and potentially consolidating commonalities between those two files as well.