Update precomputed annotation-data at run-time?

We are rendering ~100k cell centers as point annotations. This work great. (Precomputed format, only one spacial index. The comments in #227 helped a lot to set it up.)

Now we would like to visualize run-time information that we maintain external of neuroglancer (still in javascript). Basically a integer per cell. (More concrete: For visualizing multiple external selection states via point annotation shader.)

Conceptually, I think a Uint64Map (Annotation-ID --> Uint64) would work. The data is updated only infrequently (not per-frame), so I don't think it needs to be in the shader. A callback with the AnnotationID for each to-be-rendered annotation would probably work, too, as an API.

To see where this goes, we have added a uint32 property to the precomputed annotations. We can use that in the shader, and we have made an ugly hack (in neuroglancer/annotation/renderlayer.ts, _drawGeometryChunkData_) to overwrite the already fetched precomputed data at run-time and trigger _redrawNeeded_. This works.

Are there maybe alternatives approaches for this that I've overlooked, or even existing features that could serve this use-case?

(We first tried just to link via segmentation layer / relationship. But it seems that for 90k selected segments we would get 90k http range-requests, which I'm pretty sure will have very bad performance.)