the tileQueue is not always cleared, for instance when adding a layer in GeoExplorer and then switching base layers, so make sure we also clear the tileQueue when the visibility of a layer changes
Merge pull request #876 from bartvde/tilemgr-changelayer

the tileQueue is not always cleared, for instance when adding a layer in... (r=@ahocevar)
Merge branch 'master' of github.com:openlayers/openlayers into dimNG