Scale tile queue according to number of sources
Merge pull request #655 from twpayne/scale-tile-queue

Scale tile queue according to number of sources
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
Optimize canvas renderer.

Only use setTransform when it's needed (ie. when the view is rotated)
Merge pull request #662 from fredj/canvas-draw-image-perf

Possible canvas performance enhancement
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
Redefine ol.Pixel to be Array.<number>