Add camera mirroring

Add --video-source=camera, and related options:
 - --camera-id=<id>: select the camera by its id (see --list-cameras);
 - --camera-size=<width>x<height>: select the capture size.

Fixed #241 <https://github.com/Genymobile/scrcpy/issues/241>
PR #4213 <https://github.com/Genymobile/scrcpy/pull/4213>

Co-authored-by: Romain Vimont <rom@rom1v.com>
Signed-off-by: Romain Vimont <rom@rom1v.com>