Improve test of WebGL support.

Use the same options for WebGL context creation that Cesium itself uses.
This avoids the potential for National Maps test to succeed only for
Cesium's actual creation to fail.
Add a distance legend.
Do less DOM manipulation while updating the distance legend.
Move the distance legend below the lat/lon display.
Merge remote-tracking branch 'origin/matchContextCreationOptions' into refactor
Merge remote-tracking branch 'origin/distanceLegend' into refactor