Removing imageOffset and using getImageSize.

This fixes a regression that was introduced with the Tile.Image overhaul. See http://trac.osgeo.org/openlayers/ticket/3625.

tests/Layer.html

@@ -764,7 +764,7 @@
     }
       
     function test_layer_setTileSize(t) {
-        t.plan(6);
+        t.plan(4);
 
         layer = new OpenLayers.Layer();
         
@@ -784,7 +784,6 @@
         var size = new OpenLayers.Size(2,2);
         layer.setTileSize(size);
         t.ok(layer.tileSize.equals(size), "size paramater set correctly to layer's tile size");
-        t.ok(layer.imageOffset == null, "imageOffset and imageSize null when no gutters")
       
       //set on layer
         layer.tileSize = layerTileSize;
@@ -803,10 +802,8 @@
         size = new OpenLayers.Size(10,100);
         layer.setTileSize(size);
 
-        var desiredImageOffset = new OpenLayers.Pixel(-15, -15); 
         var desiredImageSize = new OpenLayers.Size(40, 130); 
 
-        t.ok(layer.imageOffset.equals(desiredImageOffset), "image offset correctly calculated");
         t.ok(layer.imageSize.equals(desiredImageSize), "image size correctly calculated");
     }