Add --video-source=camera, and related options:
- --camera-id=ID: select the camera (ids are listed by --list-cameras);
- --camera-size=WIDTHxHEIGHT: select the capture size.
Co-authored-by: Romain Vimont <rom@rom1v.com>
Signed-off-by: Romain Vimont <rom@rom1v.com>