Transition in/out points

For my application, which shows a lot of points but allows you to zoom in and out of clusters, it would be really helpful to add and remove points. Currently I'm just hiding some, but it isn't very efficient because I'm always rendering the whole set.

I think the important thing here is giving each point a `key` that can be used to compare it to other points to know which are coming in and out, like how React does it.

No files, tests, or entry points are named. Start by locating the point and regl buffer update entry points described by the issue, then determine how keyed points should be compared for additions and removals. Done means points can be added or removed without rendering the full set.

Written by the indexing model from the issue text.