Add a way to highlight selected points, not tracks

[aganders3]

We should be able to highlight individual points as they are selected (this works currently) and have the selection follows those points through time along their tracks (this is not implemented). Ideally we would be able to modify multiple properties of the points - color, size, shape, etc.

This is something we want for visualizing the ink-style virtual experiments. It also may be something we start to visualize while tracks are still loading.

This was effectively "free" when I was calculating the track highlights on the CPU early in #42. However, with highlights calculated in the shader we no longer know which point on the track is at the current timepoint.

Setting the track length quite short kind of works, but visually is not as nice as showing an actual point.

My first thought is we need to store these in a new data structure on the PointCanvas. I am thinking an array with length numTimes, where each entry in the array would be a list of pointIDs to highlight, or a list of highlights with ID and/or location, plus addition attributes.

So something like this:

class Selection {
    color: Color;
    size: number;
    ids: number[];
    coords: Float32Array;
}

const selections: Selection[] = Array.from({length: numTimes}, () => new Selection());

I think this would have to be filled in as part of addTrack?

Then when the time slider changes, we have a couple options:

1. render points in a new Points object, separately from the regular PointCanvas.points
2. use the ids to modify the properties of individual points in PointCanvas.points
3. something else?

[andy-sweet]

I'm currently fighting with React about how to use our current effects to populate the existing selection and trigger the fetch/renders in the order that guarantees their display.

My instinct is that storing the selection of the points canvas and allowing it to be set directly (rather than through the react dispatcher and associated effect) could help, but I don't trust my instinct when it comes to React yet...

[aganders3]

Yeah this has to be held by PointCanvas for rendering anyway, so maybe just skip the React state if that makes it easier.