QMediaPlayer: If you select "Full screen" in File browser, it may show black screen.

How to reproduce:
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1. Install all the needed Qt-related libaries then install the fluidlauncher sis package.
2. Launch the QMediaPlayer application.
3. Tap file browser and select "Open File".
4. Select a MP4 to player randomly.
5. Tap File Browser and select "Full screen video" when MP4 is playing.

Expected result:
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MP4 will be showed in Full screen mode.

Actual result:
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The screen only show black background.

In private JIRA:https://qtrequirements.troll.no/browse/QT-5213