fftools/ffmpeg_filter: stop accessing OutputStream.[max_]frame_rate

Pass them to ofilter_bind_ost() via OutputFilterOptions, as is done for
most other data it needs. OutputStream.[max_]frame_rate/force_fps are no
longer used outside of ffmpeg_mux*, and so can be made private.

This is a step toward decoupling encoders from muxers.

fftools/ffmpeg_mux.h

@@ -85,6 +85,10 @@ typedef struct MuxStream {
     int             ts_drop;
 #endif
 
+    AVRational      frame_rate;
+    AVRational      max_frame_rate;
+    int             force_fps;
+
     const char     *apad;
 } MuxStream;