The relative mouse mode is tracked by SDL, and accessible via
SDL_GetRelativeMouseMode().
This is more robust in case SDL changes the relative mouse mode on its
own.
Add an option --otg to run scrcpy with only physical keyboard and mouse
simulation (HID over AOA), without mirroring and without requiring adb.
To avoid adding complexity into the scrcpy initialization and screen
implementation, OTG mode is implemented totally separately, with a
separate window.
PR #2974 <https://github.com/Genymobile/scrcpy/pull/2974>
