Automatically select audio source

If --audio-source is not specified, select the default value
according to the video source:
 - for display mirroring, use device audio by default;
 - for camera mirroring, use microphone by default.

app/src/options.h

@@ -50,6 +50,7 @@ enum sc_video_source {
 };
 
 enum sc_audio_source {
+    SC_AUDIO_SOURCE_AUTO, // OUTPUT for video DISPLAY, MIC for video CAMERA
     SC_AUDIO_SOURCE_OUTPUT,
     SC_AUDIO_SOURCE_MIC,
 };