Fix CesiumWidget render loop

`scene.initializeFrame` needs to be called before anything else happens.  By calling `clock.tick` first, listeners to the tick even were seeing the system in a mismatched state (i.e. the camera could think you were in one scene mode while the scene thought you were in another.  This manifested itself in an easy to reproduce crash:

1. Load simple.czml in Viewer
2. Click on a satellite and follow it.
3. Morph to another mode and click to cancel the morph halfway through
4. Crash.

This change fixes that issue.
Merge pull request #956 from AnalyticalGraphicsInc/fix-viewer-crash

Fix CesiumWidget render loop