Initialize cached widget sizes to undefined.

If the canvas is never resized after creation of a Cesium widget, we would never render.  Rather than caching canvas size at creation time, this ensures that we always run through the resize code at least once.
Use 0 instead of undefined for initial widget size.

Viewer needs lastWidth to always have a valid value and using 0.
Merge pull request #1547 from AnalyticalGraphicsInc/fix-resize

Initialize cached widget sizes to undefined.