TilesRendererBase: Add "loadParents" flag for "optimizedLoadStrategy" traversal

[gkjohnson]

Related to #1426

Adds TilesRenderer.loadParentTiles flag to switch to loading parent tiles when optimizedLoadStrategy is true. This will allow for both types of load behavior with a single set of traversal functions.

TODO

• Review
• Test
• Clean up
• Resolve what happens when loadSiblings & loadParents are set to true
• Consider making loadParents behave more like the legacy strategy
  • parents should be leaf tiles / active if the child tiles are not loaded, yet.
  • loadSiblings and loadParents cannot be set to true simultaneously
• Determine what to do when the cache is full in order to avoid tile gaps.
  • also if the cache is full then the low LoD tiles will never be unloaded.
  • Only a problem with optimized strategy

Next

• Mark "optimizedLoadStrategy" as true by default, log deprecation warning when setting it to "false".