bugfix: mac: don't override file pix_fmt
This affects Mac only.
A few months ago, we found we needed to add pix_fmt for screensharing on Mac. This was mistakenly also applied to files, which was incorrect. Now we apply manual pix_fmt only to webcam and screenshare.
May need future additional correction:
- what about OS 11 Big Sur? still needed for screenshare?
- also needed for Webcam?