Comment 3 for bug 579471

Following OSM's terms:
"Bulk downloading is strongly discouraged. Do not download tiles unnecessarily."

What we want to do here is between a classic usage (download when we need it in real time) and a bulk download (download an entire zone): the tiles we want to preload will be downloaded anyway while driving.

We certainly shouldn't preload all 18-17-16-15(...) zoom levels but do it wisely and preload only the tiles that will be very necessary.

So in order to optimize the preloads, we should resolve:
- issue # 579470 (autozoom following the actual or predicted speed),
- issue # 579472 (display extrapoled tiles on nearby zooms),
before this one.