Loaded tracks disappearing

[Donald Douwsma]

I've been having problems with loaded tracks disappearing from the screen.
This seems to happen when the screen needs to be repainted (scrolled or zoomed).
The tracks usually reappear when scrolled again.

The best way I've found to reproduce this is to zoom on a section of map that
requires a tile to be downloaded. The track is displayed on a blank background
until the tile finishes downloading then disappears during the redraw.

I think this is a general race between the track being drawn and the tiles loaded.