fftools/ffmpeg_enc: do not set AVStream timebase directly

Instead, pass the encoder context to of_stream_init() and have the muxer
take the timebase from there. Note that the muxer can currently access
the codec context directly, but that will change in future commits.

This is a step towards decoupling encoders from muxers.

fftools/ffmpeg.h

@@ -871,7 +871,8 @@ int enc_loopback(Encoder *enc);
  *
  * Open the muxer once all the streams have been initialized.
  */
-int of_stream_init(OutputFile *of, OutputStream *ost);
+int of_stream_init(OutputFile *of, OutputStream *ost,
+                   const AVCodecContext *enc_ctx);
 int of_write_trailer(OutputFile *of);
 int of_open(const OptionsContext *o, const char *filename, Scheduler *sch);
 void of_free(OutputFile **pof);

fftools/ffmpeg_enc.c

@@ -367,11 +367,7 @@ int enc_open(void *opaque, const AVFrame *frame)
         return ret;
     }
 
-    // copy timebase while removing common factors
-    if (ost->st->time_base.num <= 0 || ost->st->time_base.den <= 0)
-        ost->st->time_base = av_add_q(ost->enc_ctx->time_base, (AVRational){0, 1});
-
-    ret = of_stream_init(of, ost);
+    ret = of_stream_init(of, ost, enc_ctx);
     if (ret < 0)
         return ret;
 

fftools/ffmpeg_mux.c

@@ -608,12 +608,19 @@ static int bsf_init(MuxStream *ms)
     return 0;
 }
 
-int of_stream_init(OutputFile *of, OutputStream *ost)
+int of_stream_init(OutputFile *of, OutputStream *ost,
+                   const AVCodecContext *enc_ctx)
 {
     Muxer *mux = mux_from_of(of);
     MuxStream *ms = ms_from_ost(ost);
     int ret;
 
+    if (enc_ctx) {
+        // use upstream time base unless it has been overridden previously
+        if (ost->st->time_base.num <= 0 || ost->st->time_base.den <= 0)
+            ost->st->time_base = av_add_q(enc_ctx->time_base, (AVRational){0, 1});
+    }
+
     /* initialize bitstream filters for the output stream
      * needs to be done here, because the codec id for streamcopy is not
      * known until now */

fftools/ffmpeg_mux_init.c

@@ -3385,7 +3385,7 @@ int of_open(const OptionsContext *o, const char *filename, Scheduler *sch)
         OutputStream *ost = of->streams[i];
 
         if (!ost->enc) {
-            err = of_stream_init(of, ost);
+            err = of_stream_init(of, ost, NULL);
             if (err < 0)
                 return err;
         }