Write header file with correct extradata

When recording, the header must be written with extradata set to the
content of the very first packet.

Suggested-by: Steve Lhomme <robux4@ycbcr.xyz>

Fixes <https://github.com/Genymobile/scrcpy/issues/351>
Fixes <https://github.com/Genymobile/scrcpy/issues/416>

app/src/recorder.h

@@ -10,6 +10,7 @@ struct recorder {
     char *filename;
     AVFormatContext *ctx;
     struct size declared_frame_size;
+    SDL_bool header_written;
 };
 
 SDL_bool recorder_init(struct recorder *recoder, const char *filename,