fftools/ffmpeg_filter: pass ts offset through OutputFilterOptions

Reduces the need to access OutputFile, which will allow decoupling
filtering from encoding in future commits.

fftools/ffmpeg.h

@@ -266,6 +266,8 @@ typedef struct InputFilterOptions {
 typedef struct OutputFilterOptions {
     // Codec used for encoding, may be NULL
     const AVCodec      *enc;
+
+    int64_t             ts_offset;
 } OutputFilterOptions;
 
 typedef struct InputFilter {

fftools/ffmpeg_filter.c

@@ -766,7 +766,6 @@ int ofilter_bind_ost(OutputFilter *ofilter, OutputStream *ost,
                      unsigned sched_idx_enc,
                      const OutputFilterOptions *opts)
 {
-    const OutputFile  *of = ost->file;
     OutputFilterPriv *ofp = ofp_from_ofilter(ofilter);
     FilterGraph  *fg = ofilter->graph;
     FilterGraphPriv *fgp = fgp_from_fg(fg);
@@ -778,7 +777,7 @@ int ofilter_bind_ost(OutputFilter *ofilter, OutputStream *ost,
     ofilter->ost = ost;
     av_freep(&ofilter->linklabel);
 
-    ofp->ts_offset     = of->start_time == AV_NOPTS_VALUE ? 0 : of->start_time;
+    ofp->ts_offset    = opts->ts_offset;
     ofp->enc_timebase = ost->enc_timebase;
 
     switch (ost->enc_ctx->codec_type) {

fftools/ffmpeg_mux_init.c

@@ -1373,6 +1373,8 @@ static int ost_add(Muxer *mux, const OptionsContext *o, enum AVMediaType type,
         (type == AVMEDIA_TYPE_VIDEO || type == AVMEDIA_TYPE_AUDIO)) {
         OutputFilterOptions opts = {
             .enc = enc,
+            .ts_offset = mux->of.start_time == AV_NOPTS_VALUE ?
+                         0 : mux->of.start_time,
         };
 
         if (ofilter) {