runCommandAsync
Queues command to run asynchronously, in order with the other async calls (texture and buffer uploads), and returns an ID for cancelAsyncCall. Meant for resource preparation such as asset loading; flooding it delays those uploads. onComplete runs once on the main thread (see pumpMessageQueues): AsyncCallStatus.COMPLETED if command ran, AsyncCallStatus.CANCELED if it never did. Needs isAsynchronousModeEnabled.