TilesRenderer: Make optimized traversal the default

[gkjohnson]

Related to #1421

To Confirm / Fix

• Test with a more complex additive tile set
• Fix tile splitting for QuantizedMeshPlugin, ImageOverlaysPlugin
  • See TilesRenderer: Add support for "virtual" child traversal to avoid unnecessary memory usage #857
  • "Virtual" or "hasVirtualChildren" can be added to a separate fields which can be used to determine whether tiles should be marked as used until the necessary number of children have been disposed, at least. TilesRenderer: Add concept of "virtual" tiles, support additive tile splitting #1498
• Expose variable indicating whether the tile set has full coverage or not
  • See OptimizedTraversal: Add initial "coverage" calculation #1452
  • See TilesRendererBase: Add additional loading flags #1445
  • _allChildrenReady may almost be acceptable but it seems to switch to false even when all tiles are rendering.
• Add option so the "optimized" path behaves like the original traversal approach. "loadParents" option, for example.

Future

• Fix auto-expanding children (QuantizedMeshPlugin, ImageFormatPlugins)
  • See QuantizedMesh: skip loading low resolution tiles when camera initialised close to mesh #1344
  • These do not work since they only expand the children once the tile itself is loaded and disposed of once the internal parents are disposed. That means we can get into an internal oscillation state when the parents are disposed, subsequently disposing of the children, causing that parent to render again.
  • Could implicitly add a tile with "unrenderable content" at some given number of layers so they re added / disposed in the same logic as internal tile sets. This won't work for tile splitting, though.
  • Could also adjust tiles to expand on "preprocess" or custom "expand virtual" callback and then wait for the tile to load before traversing to or loading children.
• Consider more flags
  • Add a flag indicating whether "full coverage" has been achieved during loading such that there will be no gaps even when moving the camera. A "loadProgress"-esque fields indicating a percentage of coverage filled may be better for initialization.
  • The priority download / parse queues could be adjusted to prioritize processing tiles that will fill gaps first over loading tiles that already have deeper children (or parents) loaded.

Migration

• Change "optimizedLoadStrategy" to "true" by default with a deprecation warning first
• Remove previous load strategy option