In the previous preview code, when possible, a non-blocking `to`
operation is performed and, immediately after, the output tensor is used
to create an image. If this non-blocking operation has not completed,
PIL makes a copy of the uninitialized memory to produce an image.
Generally, this will either contain zeros, or the result of a previously
generated preview. This results in both incorrect output, and wasted
computation (unless the memory this output was eventually copied to is
reallocated and displayed instead of a future preview).
To resolve this, the state of the preview generation is tracked with an
event.
- The PIL image is created with no copy
- The preview image is not sent to from the server until ready
- Completion of this event is polled with a reasonably slow frequency
- A new preview is not created if a previous preview has not completed
