Fix zoom animations and tile mispositioning in FF by adding an invisible border at the bottom of all tiles.

Seems like zoom animations problems are reproducible only on some systems, I didn't manage to find a reason for that yet.
Tile mispositioning (about 1 px gaps between tiles) is visible on the same systems while performing a zoom animation.
Another way to reproduce tile mispositioning is to zoom the whole page using browser zoom, this seems to work on all systems.