Optional ReadonlyonCalled once when a worker (or the calling thread) takes this build, after it waited in the queue.
Optional ReadonlysignalCancels the build. A build still waiting for a worker never starts; one
already running finishes in its worker (a tinybvh build can't be
interrupted) and its result is dropped. Either way the promise rejects
with an AbortError at once.
Optional ReadonlytransferOverrides the pool's transferInput for this build.
Per-build options of BvhWorkerPool.build, besides the tree's own.