Fix FF zoom animations.

Fix FF zoom animations by adding an invisible border at the bottom of all tiles.

Without this patch the top tile row is misplaced while performing a zoom animation in FF.
Could be reproduced on http://osm.org, http://leafletjs.com/, and with Leaflet v 0.5.1 in Firefox 21.0. I observe that bug every time, but if you do not know what I'm talking about, I'm attaching a screenshot:
![85](https://f.cloud.github.com/assets/291301/553977/d79508fe-c398-11e2-9f3e-21713ab64f92.png).

I'm not sure how or why this hack works, but it solves the problem for me (with Firefox 21.0). No other effect should be done.

Probably this should be tested on earlier FF versions.