Resolve "Faceless geometries are not supported" error with animations

[fedarko]

From biocore/empress#319:

Problem 2

I sometimes get an error in the console that THREE.DirectGeometry: Faceless geometries are not supported. I can only seem to reproduce this with days-since-experiment-start as the gradient when trajectory is the trajectory, bizarrely: [...]

This is an issue with THREE.js the most recent build that emperor uses raises this error, with no "visible" consequences. I haven't been able to figure out what's going on here. I think we just need to refactor the way Emperor handles noop cycles for asynchronous trajectories.

Since the user shouldn't see this error (and since it doesn't seem to affect anything) it's not a huge deal, but documenting here for reference.