Allow overwriting default ffmpeg flags

[fpetrakov]

Problem:
I want to overwrite default ffmpeg qscale flag when taking screenshots, so I can reduce screenshots size and save up some disk space and time when syncing anki

What I've done:

What I've expected to get:
Custom qscale flag overwrites the default one

What I've got after pressing "plus" export button:

I guess, the problem is that ffmpeg gets two same qscale flags instead of overwriting one with the other and, as a result, doesn't save screenshot, which leads to the error from the screenshot above

[fpetrakov]

https://github.com/animebook/animebook.github.io/blob/master/extension/bg/js/ffmpeg_commands.js#L76

[soamsy]

Didn't realize there was interest in this. You'd need a code change to get around this. Presumably adding another setting to configure a Variable Bit Rate from 1 to 31, and using that to decide the qscale would do the trick. If you or someone else has the bandwidth to make a pull request, I wouldn't mind merging a fix in.

Also, my average animebook screenshot size is <100KB, so if you make 10000 flashcards, that puts you at around 1 GB. This doesn't cause problems for me, but I can understand that being an issue.