fix video restarting on same bg #2882

[slavaGanzin]

Fixes: #2882
Alternative to: #3189

Before:

before.mp4

After:

after.mp4

This fixes only transition between fragments, between slides it's more complicated and should be handled by somebody with deep knowledge of internals like @hakimel himself :)

[hakimel]

Hey! Support for continuous background video playback across slides was added in a29a9c7

I've been testing showing and hiding fragments but haven't seen them have any effect on inline or background video playback. Can you check if you're still experiencing this in the latest version? If you are, I'd really appreciate an example presentation where this can be reproduced. Appreciate it!