mediaplayerexample shows green player item on stm32mp1

Running the mediaplayerexample on the stm32mp1-dk2, the VideoOutput item stays green.

Observations:

• independent of the backend: it happens with both ffmpeg and gstreamer
• decoding Qt_LogoMergeEffect.mp4 seems to be a bottleneck on the device (decoding via a command line player to /dev/null takes longer than the playing time of the video). though i'd expect dropped frames rather than a green item