Traversal: Improve traversal performance

[gkjohnson]

Related to #669

• Merge some of the functions. Like at least markVisibleTiles and markUsedSetLeaves.
• Accumulate toggled tiles into a set or array so it can be iterated once rather than unnecessarily traversing through all parents.
  • Timing during tests in aerial example after load completes
    • Mark Used Tiles: ~3.1ms
    • Mark Used Set Leaves: ~0.35 ms
    • Mark Visible Tiles: ~0.1 ms
    • Toggle Tiles: ~0.25 ms
• Lazily generate the error value & frustum state as-needed since it isn't always used.
  • Frustum state provides little to no benefit

Related goal: improve performance of loading / traversing tile set from iTowns/itowns#2335