It's possible to initiate an asynchronous image capture
(via AVCaptureStillImageOutput object) and immediately
stop AVCaptureSession. In this case image capture's callback
can potentially block forever, waiting for a semaphore: (if)
no new frames arrive after 'stop' (on whatever thread
AVFoundation/GDC chooses), this semaphore is never released.
To avoid this we try to acquire a semaphore with a (reasonable)
timeout and report an error in case of failure. To make sure
we are not leaking a semaphore and not creating a danling pointer,
we use a QSharedPointer now.

Task-number: QTBUG-61367
Change-Id: I208cd463f843bc807b53b23ac9651aab0382775a
Reviewed-by: Christian Stromme <christian.stromme@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>