Load low resolution tiles

This PR causes tile layer renderers to preemptively load lower resolution tiles that cover the visible extent. The highest priority is given to tiles at the current zoom level. Once all these are loaded, tiles at successively lower zoom levels are loaded. This gives a better visual effect when zooming out of an initially zoomed in map (e.g. the Stamen, rotated, or Bing Maps examples).

Note that this normally quickly reduces to one tile per zoom level. For example, if you have a map at zoom level 10 with 4x4 = 16 tiles, then it will load 16/4 = 4 tiles at zoom level 9, 1 at zoom level 8, and 1 at each of zoom levels 7...0 for a total of 13 extra tiles. In the worse case, when the center of the map is at (0, 0) then four tiles are loaded per zoom level.