Render error tiles as empty
Treat image tiles with a zero dimension as empty
Return listener key from ol.Object#on
Add ol.Object#unByKey
Add ol.Object#onOnce
Only set center once in mobile full screen example
Merge pull request #658 from twpayne/ol-object-events

ol.Object events
Merge pull request #663 from twpayne/render-error-tiles-as-empty

Render error tiles as empty