Feature - continous camera movement tracking

[seankmartin]

The context can be used to mark the camera continuously moving. The context will mark the camera movement as completed after 300ms using lodash debounce, and emit a signal when this happens. At the moment, only the perspective panel listens for this signal, and uses it to trigger a redraw request if volume rendering is enabled on that panel.

The actions that flag the start of a continuous camera movement are:

• Zooming via mouse, touch, or keys
• Rotating via mouse, touch or keys
• Changing the depth planes

[seankmartin]

The changes here have been integrated into https://github.com/google/neuroglancer/pull/589, but 589 can be updated again based on changes from this PR

[seankmartin]

Closed as #591 included these changes