Fix bounce() bounce at the wrong position

The animation is fine on map load, but the markers are bouncing at the
wrong place when the user has zoomed or dragged the map and one is
calling the "bounce" method on the markers afterwards.

To get the marker position on the screen (x, y), we should use the
original latitude and longitude (don't change) and not the original
pixel coordinates (change upon zoom/drag).

Rework the commit log. Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Fix markers when map is panned.

If the user move the map while the marker are bouncing, it will bounce all over
the place.
Now, we take the movement of the map into account each time we compute the next
marker position.

Rework the commit log. Maxime Hadjinlian <maxime.hadjinlian@gmail.com>