Implicit Tiling: Consolidate redundancies in availability calculations

A quick glance at [`QuadtreeAvailability.cpp`](https://github.com/CesiumGS/cesium-native/blob/de933c29dd36d9582059239fb166af666cf14b41/CesiumGeometry/src/QuadtreeAvailability.cpp) and [`OctreeAvailability.cpp`](https://github.com/CesiumGS/cesium-native/blob/de933c29dd36d9582059239fb166af666cf14b41/CesiumGeometry/src/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.