annotations: Presence of invlerp uicontrol causes incorrect rendering

The presence of `#uicontrol invlerp` in an annotation shader causes strange rendering artifacts.

Here's a minimal example, using the old hemibrain synapses (which were written into `precomputed` format by @jbms himself, I think).

https://neuroglancer-demo.appspot.com/#!gs://flyem-user-links/short/annotation-rendering-problem.json

Note that the `invlerp` control is not even _used_ by the actual shader function. It's mere presence triggers the issue. Another strange thing is that the issue only shows up if the "Rendering" tab is selected. If you click away to "Source" or "Annotations", then rendering becomes correct again.

It seems as if the renderer isn't occluding points correctly. Instead of points nearer to the camera occluding points behind them, it's almost reversed, with far-away points magically occluding the closer ones. But that's not _quite_ a correct description because something about the spatial grid boundaries complicates the story.