Add --print-fps to enable FPS counter on start

The FPS counter could be enabled/disabled via MOD+i.

Add a command line option to enable it on start. This is consistent with
other features like --turn-screen-off or --fullscreen.

Fixes #468 <https://github.com/Genymobile/scrcpy/issues/468>
PR #3030 <https://github.com/Genymobile/scrcpy/pull/3030>