TilesRenderer: Improve ergonomics of "loadProgress"

[gkjohnson]

• Ensure load progress is updated before "load-model" callback
  • Requires #911, #877
• Prevent bouncing back and forth of load progress due to in progress tiles not being added to the lru cache preemptively.
  • Possibly just this function can just be adjusted to traverse all children to ensure they are loading if "used". Then all necessary tiles will be queued immediately which will be reflected in the loadProgress field- hopefully without added overhead.

[gkjohnson]

The bouncing can't really be prevented in some cases since we can't know how many tiles will need to be loaded when an external tile set is required.