maps on small devices) this is a huge win: instead of downloading
*42 tiles* on a 500x500 div, we now download just 12, a 3.5x savings
in downloads by default. The buffer of 2 was originally chosen when
most maps were based on slow-loading WMS laeyrs and having an area
to pan into was more important. That default no longer makes sense.
(Closes#3111) r=tschaub
git-svn-id: http://svn.openlayers.org/trunk/openlayers@11481 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf