Some styling changes for the BaseLayerPicker.
Silly scrollbar.
Don't refine tiles past our available data.

If a tile's geometry was created by upsampling its parent, and all of its
TileImagery instances are adopted from its parent as well, mark the tile
as UPSAMPLED_ONLY.  When selecting tiles to render, don't refine a parent
tile when all of that tile's children are marked UPSAMPLED_ONLY, because
it's just extra draw commands and GPU work with no benefit.

This reduces the number of tiles rendered in the Half Dome view in the Terrain
Sandcastle example from 243 to 219, which is still way too many.  Other
views are unaffected because they're not close enough to exceed available
data.
Cleanup, CHANGES.md, add a test, fix a test.
Past tense.
Added Kevin to individuals section
Merge pull request #1672 from AnalyticalGraphicsInc/limitRefinement

Don't refine tiles past our available data.
Use a properly invalid URL in ArcGisMapServerImageryProvider tests.
Merge pull request #1673 from AnalyticalGraphicsInc/arcgis-specs

Use a properly invalid URL in ArcGisMapServerImageryProvider tests.
CesiumWidget and Viewer improvements.
Update CHANGES.md.
Pass the timestamp to the pre- and post-render events.
Merge remote-tracking branch 'origin/master' into style-tweak
Merge pull request #1663 from AnalyticalGraphicsInc/style-tweak

Style changes for BaseLayerPicker
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`.
Remove unnecessary public function.
Use spies instead of custom code.
Factor the render func into a separate func.

This way the try/catch won't disable optimization of the entire function.
Remove redundant assetions.