SH-4645 Viewer hangs on exit after cancelling a mesh upload.

Problem involved a 3-way livelock between the main, upload
and decomposition threads.  Viewer is shutting down but an
upload is in the 'generate hulls' state.  Main thread asks
upload request to discard and spins waiting for it to finish.
Upload thread is in generateHulls spinning waiting for the
decomposition thread to process a mesh request.  Decomposition
thread is sleeping waiting for main thread to deliver work
that upload thread has asked the decomposition thread to do.

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