fixes #46, ref #75, ref #57

[Superjamp]

changes

• using the correct resolution and viewbox for the screenshots and for cursor of the slides from bigbluebutton/bbb-playback
• be able to set output width and hight and influence the quality of the screenshots Video resolution #46
• make isOnlyAudio overwritable TypeError: Cannot read property 'isOnlyAudio' of null #57
• make stackWebcamsToPresentation skippable
• be able to set ffmpeg threads
• change the node docker version ffmpeg Rendering giving error #75

[tilmanmoser]

Thanks a lot!