Make it explicit that functions operate on tile sources
Add ol.source.TileSource.useTile
Implement useTile for image tile sources with caches
Mark all low resolution tiles as used to prevent cache expiration
Move useLowResolutionTiles from layer renderer to tile source
Avoid creating a TileCoord object when marking used tiles
Merge pull request #226 from twpayne/preserve-low-resolution-tiles

Preserve low resolution tiles