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>

app/scrcpy.1

@@ -211,6 +211,10 @@ Inject alpha characters and space as text events instead of key events.
 This avoids issues when combining multiple keys to enter special characters,
 but breaks the expected behavior of alpha keys in games (typically WASD).
 
+.TP
+.B "\-\-print\-fps
+Start FPS counter, to print framerate logs to the console. It can be started or stopped at any time with MOD+i.
+
 .TP
 .BI "\-\-push\-target " path
 Set the target directory for pushing files to the device by drag & drop. It is passed as\-is to "adb push".