Disable audio on initialization error

By default, audio is enabled (--no-audio must be explicitly passed to
disable it).

However, some devices may not support audio capture (typically devices
below Android 11, or Android 11 when the shell application is not
foreground on start).

In that case, make the server notify the client to dynamically disable
audio forwarding so that it does not wait indefinitely for an audio
stream.

Also disable audio on unknown codec or missing decoder on the
client-side, for the same reasons.

PR #3757 <https://github.com/Genymobile/scrcpy/pull/3757>

app/src/recorder.h

@@ -23,6 +23,14 @@ struct sc_recorder {
     struct sc_packet_sink video_packet_sink;
     struct sc_packet_sink audio_packet_sink;
 
+    /* The audio flag is unprotected:
+     *  - it is initialized from sc_recorder_init() from the main thread;
+     *  - it may be reset once from the recorder thread if the audio is
+     *    disabled dynamically.
+     *
+     * Therefore, once the recorder thread is started, only the recorder thread
+     * may access it without data races.
+     */
     bool audio;
 
     char *filename;
@@ -42,6 +50,9 @@ struct sc_recorder {
     sc_cond stream_cond;
     const AVCodec *video_codec;
     const AVCodec *audio_codec;
+    // Instead of providing an audio_codec, the demuxer may notify that the
+    // stream is disabled if the device could not capture audio
+    bool audio_disabled;
 
     int video_stream_index;
     int audio_stream_index;