support for buffer:0 on grids, and includes changes to Remove extra
rows and columns from the grid in _initTiles when reusing an existing
grid, which is a fix to #357 and #436.
git-svn-id: http://svn.openlayers.org/trunk/openlayers@2091 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf