Darwin: propagate known video size to newly attached video sink

AVFMediaPlayer forwarded presentationSize changes to the current sink
only, so a sink attached after the media was loaded stayed without a
size. Keep the last known size and apply it in setVideoSink(), and
reset it on setMedia().

Also ignore the empty presentationSize reported before the size is
known, which caused an extra videoSizeChanged emission.

Unskip setMedia_setsVideoSinkSize_beforePlaying on Darwin.

Pick-to: 6.12
Change-Id: Ia42a6db5414d736a7ab4b821ce50cc4918a15b55
Reviewed-by: Tim Blechmann <tim.blechmann@qt.io>