Add option for custom video source

[napiorkowskimd]

• [X] I have checked that a similar feature request does not already exist.

Is your feature request related to a problem? Please describe. I use scrcpy to connect with a device that is connected to an usb capture card, and I need to use two windows to control the device (one window with scrcpy, and one to view the capture card output)

Describe the solution you'd like Add option to provide custom video source (eg --video-source=/dev/video0)

Additional context I have some initial work done on my fork https://github.com/napiorkowskimd/scrcpy/pull/1/files and I'm curious if you are interested in merging this to the main branch.