fftools/ffmpeg_filter: move most of -apad logic to the muxer

The decision whether -apad actually does anything is made based on muxer
properties, and so more properly belongs there. Filtering code only
receives the result.

fftools/ffmpeg_mux.h

@@ -78,6 +78,8 @@ typedef struct MuxStream {
 #if FFMPEG_OPT_VSYNC_DROP
     int             ts_drop;
 #endif
+
+    char           *apad;
 } MuxStream;
 
 typedef struct Muxer {