Do not fail if SDL_INIT_VIDEO fails without video

The SDL video subsystem may be initialized so that clipboard
synchronization works even without video playback.

But if the video subsystem initialization fails (e.g. because no video
device is available), consider it as an error only if video playback is
enabled.

Refs 5e59ed31352251791679e5931d7e5abf0c2d18f6
Fixes #4477 <https://github.com/Genymobile/scrcpy/issues/4477>