Map stories: Performance optimizations

[goldpbear]

Complex story transitions (with large centerpoint jumps, zooms, pans, and layer/basemap changes) are currently pretty choppy. I can think of a few optimizations though:

• Since we know the layer visibility settings of adjacent story items, preload those layers when the user is on a given story item. This includes potential basemap changes.
• Don't perform zoom style calculations (the eval statements) for layers that aren't visible.

It would be great to preload basemap tiles at a given centerpoint and zoom level, but so far I haven't found a way to do this.