audio/video sometimes get out of sync in longer videos

[t-brieger]

this only happens, or is at least only noticable, for quite long videos - over 2-3 minutes - and is probably related to how we handle audio in general.

(following paragraph copied from description of #58)
in all situations where i have so far encountered this new issue, it can be fixed by setting the source video's frame rate to exactly 25, presumably because that's what ffmpeg uses as a default frame rate for input files that don't have an inherent "speed" (like a bunch of still frames).

if this really is the reason, a simple -r ${framerate} flag to ffmpeg could possibly already fix it