Record a packet with its duration

Record a packet only once the following has been received, so that we
can set its duration before muxing it.

Fixes <https://github.com/Genymobile/scrcpy/issues/702>

app/src/recorder.h

@@ -34,6 +34,12 @@ struct recorder {
     bool stopped; // set on recorder_stop() by the stream reader
     bool failed; // set on packet write failure
     struct recorder_queue queue;
+
+    // we can write a packet only once we received the next one so that we can
+    // set its duration (next_pts - current_pts)
+    // "previous" is only accessed from the recorder thread, so it does not
+    // need to be protected by the mutex
+    struct record_packet *previous;
 };
 
 bool