look for loaded parent tiles in cache

This reduces the amount of the view not covered by tiles.
It makes tile loading look a lot faster.

It helps a lot when you:
- zoom in and the pan around
- zoom in quickly and then zoom out more slowly

ref #1918

js/source/tile_pyramid.js

@@ -195,7 +195,12 @@ TilePyramid.prototype = {
             var tile = this._tiles[coord.id];
             if (tile && tile.loaded) {
                 retain[coord.id] = true;
-                return tile;
+                return true;
+            }
+            if (this._cache.has(coord.id)) {
+                this.addTile(coord);
+                retain[coord.id] = true;
+                return true;
             }
         }
     },