Comment 5 for bug 579379

What I meant by the comment was that when I'm driving at 60km/h, not even a 3G connection can download the tiles fast enough. It's especially bad if you're driving diagonally. Willu had fixed this in #70 but I found that it tried to download too much and slowed my phone down. I think the way to fix this issue is to preload tiles only in the direction that the user is travelling.