Fix: config.doNotFlipLocalVideo not respected for virtual-backgrounds

[psi-4ward]

(closes #11553)

CLA already signed

[jitsi-jenkins]

Hi, thanks for your contribution! If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[psi-4ward]

Easy to test with a custom virtual background with some text ;)

[github-actions[bot]]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.