Using integer pixel space to avoid whitespace between tiles.

lib/OpenLayers/Layer/Grid.js

@@ -1014,12 +1014,12 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
                 });
      
                 tileoffsetlon += tilelon;       
-                tileoffsetx += tileSize.w;
+                tileoffsetx += Math.round(tileSize.w);
             } while ((tileoffsetlon <= bounds.right + tilelon * this.buffer)
                      || colidx < minCols);
              
             tileoffsetlat -= tilelat;
-            tileoffsety += tileSize.h;
+            tileoffsety += Math.round(tileSize.h);
         } while((tileoffsetlat >= bounds.bottom - tilelat * this.buffer)
                 || rowidx < minRows);