Reuse ol.TileRange object in forEachTileCoordParentTileRange
Avoid creating extra ol.Size object in getPixelBoundsForTileCoordAndResolution
Avoid creating ol.Coordinate objects to call getTileCoordForCoordAndResolution_
Make reverseIntersection argument mandatory
Replace getTile with getTileZXY

This massively reduces the number of temporary ol.TileCoord objects.

Previously an ol.TileCoord object was generated for every potentially
visible tile at the current zoom level and lower, every frame.  This
commit eliminates all of those.  Now new ol.TileCoord objects are only
allocated when a new tile is created.
Rename getTileZXY to getTile
Remove ol.TileRange.clone
Rename useTileZXY to useTile
Merge pull request #428 from twpayne/gc-optimizations

GC optimizations