fftools/ffmpeg: rework applying input -r

Do not use a separate counter for CFR timestamps forced with -r used as
an input option. Set durations properly and let estimation code do the
rest.

fftools/ffmpeg.c

@@ -985,7 +985,7 @@ static int64_t video_duration_estimate(const InputStream *ist, const AVFrame *fr
     // durations, then this should be simplified.
 
     // prefer frame duration for containers with timestamps
-    if (frame->duration > 0 && !ifile->format_nots)
+    if (frame->duration > 0 && (!ifile->format_nots || ist->framerate.num))
         return frame->duration;
 
     if (ist->dec_ctx->framerate.den && ist->dec_ctx->framerate.num) {
@@ -1090,8 +1090,12 @@ static int decode_video(InputStream *ist, const AVPacket *pkt, int *got_output,
 
     frame->pts = frame->best_effort_timestamp;
 
-    if (ist->framerate.num)
-        frame->pts = ist->cfr_next_pts++;
+    // forced fixed framerate
+    if (ist->framerate.num) {
+        frame->pts       = AV_NOPTS_VALUE;
+        frame->duration  = 1;
+        frame->time_base = av_inv_q(ist->framerate);
+    }
 
     // no timestamp available - extrapolate from previous frame duration
     if (frame->pts == AV_NOPTS_VALUE)

fftools/ffmpeg.h

@@ -372,10 +372,6 @@ typedef struct InputStream {
 
     int64_t filter_in_rescale_delta_last;
 
-    // when forcing constant input framerate through -r,
-    // this contains the pts that will be given to the next decoded frame
-    int64_t cfr_next_pts;
-
     int64_t nb_samples; /* number of samples in the last decoded audio frame before looping */
 
     AVDictionary *decoder_opts;