Some optimizations

* Tiles are now cut out of the sketch renderer in a separate
  pass. This ensures that point features at tile borders appear
  at both sides of the border. However, if such features get
  added in a later tileRange rendering pass, tiles from a
  previous rendering pass will still not have that feature.
* The tile canvas is only created once, and cloneNode(false) is
  used to get a canvas for a new tile.