WorkQueue for background task

[cahirwpz]

To allow concurrent effect data loading (with Effect.Load) and rendering (with Effect.Render) the demo must be able to delegate work to a background task. Effect.Render must not busy-wait but use TaskWait(VBlankEvent) to leave some time for concurrent execution of Effect.Load. Use cases follow:

• Initial phase of a demo:

WorkQueueT wq;
WorkT w1 = {.func = Effect1.Load};
...

WQInit(&wq);
WQAdd(&wq, &w1);
...

/* Loading screen... */
while (!WQIsEmpty(&wq)) {
  /* render frame */
  TaskWait(VBlankEvent); /* Switch out to background thread! */
}

• Before every effect we need to make sure a job is DONE. This should suspend calling task if the job is RUNNING.

WQWaitFor(&w5);
/* Do the next effect. */

• If user wants to quit demo, we need to cancel all READY jobs and wait for RUNNING job to finish.

WQDrain(&wq);