app: allow canceling line-art computation
Line-art computation can take a long time, and it's therefore
desirable for it to be interruptable. While we do cancel the line-
art async when its result is no longer needed, most parts of the
computation don't respond to the cancelation request, leaving the
async operation running in the background, blocking subsequent
async operations.
Implement cancelation support of line-art computation, by passing
down the async object to the various functions, and periodically
checking for its cancelation at various points. When the async is
canceled, we quickly abort the operation.
Even though cancelation now happens relatively quickly, some parts
of the computation are still uninterruptable and may incur some
latency, so we avoid waiting for the async opration to be aborted
after cancelation, as we did before.
(cherry picked from commit d8e69d66bc)
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