Change default circle granularity

[pjcozzi]

People often complain about the performance:

• https://groups.google.com/forum/#!topic/cesium-dev/CzkZXqX6axQ
• https://groups.google.com/forum/#!topic/cesium-dev/TXoEGD69Hrk

Let's do this for all granularity-related geometries.

A related idea is, in addition to granularity, to have something like resolution (or whatever) that is just low, medium, and high for ease of use.

[mramato]

I think the main problem is that the ellipse geometry uses many more vertices at the same granularity as a polygon does. See #1515. I think a pass on all geometries for a decent default granularity is probably good enough; but If users still run into issues after that then I would be open to trying out a resolution property. Another idea would be to add a global low/medium/high setting that affects all geometry. Users would still be able to override individual geometries by passing in a granularity, but if no granularity is supplied, one is selected based on the global low/medium/high setting.

[pjcozzi]

@bagnell can you update the forum posts:

• https://groups.google.com/forum/#!topic/cesium-dev/CzkZXqX6axQ
• https://groups.google.com/forum/#!topic/cesium-dev/TXoEGD69Hrk