Probably we should add `this.zoomRingDragging = false;` here, along with others like it.  The code works fine without it, but it's good form to declare all the fields in the constructor.  This helps with code readability, helps the JS optimizer know what to expect, and is needed for documentation.

Also, all "public" fields (anything that does not start with an underscore) should have a JSDoc-style comment on it.  In terms of GSoC deadlines, getting the code working before the end of GSoC is more important than docs, but before this can go in Cesium master it will need documentation & tests.