When the TileManager adds an image to its cache, it might be a backbuffer
image. Backbuffer images have an id with a '_bb' postfix. Regular images
do not have an id at all. In OpenLayers.Layer.Grid, in
addTileMonitoringHooks, a loadend listener is created for each tile. This
listener checks for tie image id, and will remove it from its parent node.
This will cause images to be removed from the layerDiv if the image comes
from the TileManager's tileCache and was on a backbuffer by the time it was
added to the cache.
Simply removing the image's id before assigning it to a tile resolves the
issue.
When a layer sets a cached image's className to something else than
.olTileImage (e.g. by setting .olTileReplacing), we should not keep the
image in the cache any more, because it may no longer be valid.
Having the TileManager remove an image from the DOM, then setting the
cached image, and then having to position it felt a bit awkward. With the
new beforeload event, the setImage method and putting renderTile before
positionTile, providing the cached image feels way more natural.
