In OpenLayers 2.12, only one tile was loaded per animation frame. This
change brings back something similar as part of the tile manager. But we
do not use requestAnimationFrame anymore, because @elemoine pointed out
that browsers may give requestAnimationFrame invoked functions a higher
priority than setTimeout invoked functions.
The reason for this change is
http://lists.osgeo.org/pipermail/openlayers-users/2013-April/028063.html.
