Darwin: don't re-emit unchanged video size during playback

AVPlayerItem re-reports the same presentationSize once playback
starts. Forwarding it to the sink emitted videoSizeChanged without a
corresponding video frame. Only propagate actual size changes.

Unskip videoSinkSignals on Darwin.

Pick-to: 6.12
Change-Id: Ib37f4f7e6824231adccd1979a8218935e864aabb
Reviewed-by: Tim Blechmann <tim.blechmann@qt.io>