Factor out core elements of ol.TileQueue into ol.structs.PriorityQueue
Refactor ol.TileQueue to extend ol.structs.PriorityQueue
Replace isHeap with assertValid
Add optional per-tile callback to manageTilePyramid
Store focus in frame state
Add tile texture queue
Use tile texture queue
Tune tile priority functions
Make tile pre-loading a per-layer option
Add preload example
Set preload appropriately in examples
Make preload a number of levels instead of a boolean