fftools/ffmpeg: add an AVClass to Muxer/OutputFile

Use it for logging. This makes log messages related to this output file
more consistent.

fftools/ffmpeg_mux.h

@@ -63,6 +63,9 @@ typedef struct MuxStream {
 typedef struct Muxer {
     OutputFile of;
 
+    // name used for logging
+    char log_name[32];
+
     AVFormatContext *fc;
 
     pthread_t    thread;