White flash occurs when zooming out from initially zoomed in view with WebGL renderer

To reproduce:

1. Start with a zoomed in view (e.g. Stamen, rotated, or canvas tiles example) with the WebGL renderer
2. Zoom out
3. Observe white flash as a the map is temporarily rendered with no tiles

#357 preemptively loads tiles at lower zoom levels. This means that tiles should always be available and no white flash should occur.

This also occurs with the Bing Maps example with both the Canvas and WebGL renderer, although this might be a different bug.