Rename --no-mirror to --no-playback

This option impacts video and audio _playback_. For example, if we use
V4L2, the device is still "mirrored" (via V4L2), even if playback is
disabled. Therefore, "playback" is more approriate than "mirror".

The initial option --no-display option was renamed to --no-mirror by
commit 6928acdeac, but this has never been
released, so it is ok to rename it one more time.

Refs #3978 <https://github.com/Genymobile/scrcpy/pull/3978#issuecomment-1549420103>
PR #4033 <https://github.com/Genymobile/scrcpy/pull/4033>

app/src/options.c

@@ -52,7 +52,7 @@ const struct scrcpy_options scrcpy_options_default = {
     .fullscreen = false,
     .always_on_top = false,
     .control = true,
-    .mirror = true,
+    .playback = true,
     .turn_screen_off = false,
     .key_inject_mode = SC_KEY_INJECT_MODE_MIXED,
     .window_borderless = false,