/usr/bin/grab returns all-black image on subsequent calls

[wedebe]

Originally logged at openatv/enigma2#2369

Pause broadcast/playback

Capture screenshot by either

using the OpenWebif url /grab?format=png&mode=all
or
from shell on e2 box, run /usr/bin/grab
view image (whatever frame video is currently paused at)

now repeat same step to capture screenshot

view image (now all-black)

Expected: whatever frame video is currently paused at should be saved again, no matter how many times a screenshot is saved while the image remains the same on-screen.

I tried to work around this by adjusting e2's brightness, only to realise that process happens at a later stage.

Just to add: even showing OSD and screenshotting will capture OSD on an all-black background.