DirectShow: (Optical) Zoom is changed by DirectShowCameraZoomControl even when not requested

DirectShowCameraZoomControl caches currently requested (optical) zoom in _m_requestedOpticalZoom_ and tries to restore it each time, when QCamera is being in Loaded state.

The problem happens , if zoom was not requested externally at all. (_m_requestedOpticalZoom_ == 0) - in that case DirectShowCameraZoomControl shouldn't change zoom level at all.

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User use case:
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I'm using a webcam that supports optical zoom and has a (physical) remote to change it.
I have a Qt application (currently using Qt 5.11.2) that streams from that camera.
I'm using the remote (only) to change zoom level.
Sometime I do a resolution change (stop steam -> change resolution -> start stream), and I expect Zoom level to remain the same

In fact - zoom is always being reset to 1x, which is inconvenient.

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Didn't happen in Qt 5.9 (it didn't have DirectShowCameraZoomControl at all).
Does not happen on macOS (AVFCameraZoomControl has different behavior) no zoom control there either.

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Suggested fix:
In this line of code also check that _m_requestedOpticalZoom_ is non-zero.