v4l2 and libcamera fallback-only options

Julian Bouzas requested to merge julian/wireplumber:v4l2-default-option into master

This MR adds an option in V4L2 monitor to not be created if libcamera monitor can be created, and another option in libcamera monitor to not be created if v4l2 monitor can be created.

This allows users to prioritize either video backend if one is not installed.

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