Bound of UncapturedErrorHandler is more restrictive than it should be

[kpreid]

The definition of UncapturedErrorHandler, the callback given to Device::on_uncaptured_error(), is an alias for

Fn(Error) + Send + 'static

The combination of Fn, Send, and no Sync is weird. I believe that it should be replaced with either:

Fn(Error) + Send + Sync + 'static

implying that the callback could be called simultaneously from multiple threads, where it currently cannot*, or

FnMut(Error) + Send + 'static

allowing the callback to take advantage of not being called simultaneously. This is not a soundness issue, just an unnecessary empty space in between what is permitted for wgpu and what is permitted for its callers.

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* Say, what happens if the uncaptured error handler itself performs a wgpu operation that errors? Deadlock? Possibly the error handler ought to be in an Arc so it can be called outside the lock.