TilesRenderer: Add configurable delay to downloads

[gkjohnson]

This can be done by just adding something like a "wait" before actually triggering a download. This will provide the opportunity to cancel loads if they are no longer needed or the camera is moving quickly (see #1422). This potentially causes extra delays, though, if a tile is in the queue and then removed and then added again in cases where it may have just been unused for one frame. Can this be made more resilient?

[gkjohnson]

The delay should be on-the-fly update-able such that it retroactively applies to all queued tiles. Adding something like a "DelayedQueue" to trigger callbacks after a certain time period may be best. A single "setTimeout" handle can be kept allowing for an easily update-able delay.

[gkjohnson]

Some other strategies:

• Track the velocity of the camera movement and scale the "delay" based on the camera movement.
• Don't load tiles (or load fewer tiles based on something like position in camera view) while the camera is moving.
• Cancel downloading tiles during camera move. If the camera is moving we can cancel downloads that may not be necessary based on tile size / direction