Pass the timestamp to the pre- and post-render events.
Merge remote-tracking branch 'origin/master' into widgetAndViewerImprovements
Move render error events to Scene.

Also Viewer no longer has its own render loop, it just uses the one in
`CesiumWidget`.
Merge branch 'widgetAndViewerImprovements' into performanceMonitorWidget

Conflicts:
	Source/Widgets/CesiumWidget/CesiumWidget.js
	Source/Widgets/Viewer/Viewer.js
Fix bad merge.