One layer of a map, and the thing the renderer actually draws.
A layer and not the map, because the order things are drawn in is per drawable and a layer's whole
purpose is to sort on its own: the ground under the player, the treetops over them, both from one
map. Making the map one drawable would have meant teaching the sorter about an order inside an
order, when the one it has already does the job.
Its cells are split across two batches. The still one holds every cell whose tile shows a
fixed picture, and is built once. The moving one holds the cells whose tile cycles, and only their
corners are rewritten when the cycle turns. That split is what keeps a map with water cheap: a few
dozen cells go up again four times a second while the several thousand still ones sit there.
Those two batches are not here. They are handles to memory on a device, and this is a record:
plain data, and all of it writable to a file. They live in a side table instead, keyed by the
layer, which is the same arrangement a particle emitter's pool uses. See layer_state.ts.