Sphere widget behaves better when scaled down to zero

Radius was becoming non-determinate and the widget could not be scaled
back from zero. Added new test to check zoom.

Change-Id: Ib308bc7109d6624fbf3ff725bae33ecc93e45b72
Merge topic '0014323-sphere-widget-scale' into master

7e09ea95 Sphere widget behaves better when scaled down to zero