Add resolutionScaleFactor to CesiumWidget and Viewer.
Force a resize when scale-factor changes.
Fixes after review.

1. No need to have forceResize property in Viewer.
2. Fix typo in CesiumWidget.
Use property setter.
First cut at resolutionScale uniform.

Still need doc updates and cleanup.
Ongoing resolution scale work.
Merge remote-tracking branch 'origin/master' into resolutionScaleFactor

Conflicts:
	Source/Widgets/CesiumWidget/CesiumWidget.js
Simplify resolutionScale by removing it from the scene and computing it automatically in UniformState.update.
Fix billboards
Update CHANGES.
Revert no longer needed changes.
Add specs.
Merge remote-tracking branch 'origin/master' into resolutionScaleFactor
Merge remote-tracking branch 'origin/master' into resolutionScaleFactor

Conflicts:
	CHANGES.md
Merge remote-tracking branch 'origin/master' into resolutionScaleFactor
Changes after review

`UniformState.resolutionScale` should be read-only.
Merge remote-tracking branch 'origin/master' into resolutionScaleFactor

Conflicts:
	Source/Renderer/UniformState.js
	Source/Scene/Billboard.js
Fixes after merge.
Merge pull request #1678 from AnalyticalGraphicsInc/resolutionScaleFactor

Add resolutionScale to CesiumWidget and Viewer.