The 'mirrored' property indicates the QVideoFrames need to be mirrored
along their vertical axis. This is typically needed for video frames
coming from a front camera on a mobile device.
This is implemented as a string-based property. In Qt 5.6, this should
be replaced by a new public function.
Change-Id: Ideb7de81e83f66826f4efb5f2951c4beec13546b
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>
3c54acb6f7