Backbuffer and transitionend only when there are tiles

When an application zooms the map programmatically right after
initialization, there will be an empty backbuffer. This should be avoided,
because it adds an unnecessary DOM element.

The tile.onLoadEnd handler registers a transitionend listener on the image
of the last loaded tile. But if loading was aborted, there will be no
image. The new logic registers the transitionend event on the last child
of the layer div, which is not necessarily the tile we're handling loadend
for. When the backbuffer is empty (i.e. no tile was loaded at the time the
backbuffer was created), it will be removed immediately.

tests/Layer/Grid.html

@@ -1334,7 +1334,9 @@
         t.ok(layer.backBuffer === map.layerContainerDiv.firstChild,
              '[a] back buffer is first child of layer container div');
 
-        // Mark one tile loaded, to see if back buffer removal gets scheduled.
+        // Mark one tile loaded and add an element to the backbuffer, to see if
+        // backbuffer removal gets scheduled.
+        layer.backBuffer.appendChild(document.createElement('img'));
         layer.grid[1][1].onImageLoad();
 
         t.ok(layer.backBufferTimerId !== null,