Always force transcoding if "Optical" device type is selected

If you force the output channel layout to something other than stereo,
AC3 passthrough can not work for technical reasons.

This was causing major user confusion, especially because the channel
selection is hidden if the "Optical" device type is selected.

src/player/PlayerComponent.cpp

@@ -702,6 +702,11 @@ void PlayerComponent::setAudioConfiguration()
 
   // set the channel layout
   QVariant layout = SettingsComponent::Get().value(SETTINGS_SECTION_AUDIO, "channels");
+
+  // always force either stereo or transcoding
+  if (deviceType == AUDIO_DEVICE_TYPE_SPDIF)
+    layout = "2.0";
+
   mpv::qt::set_option_variant(m_mpv, "audio-channels", layout);
 
   // if the user has indicated that PCM only works for stereo, and that