video: use demuxer-signaled duration for last video frame

Helps with gif, probably does unwanted things with other formats.

This doesn't handle --end quite correctly, but this could be added
later.

Fixes #3924.

player/core.h

@@ -353,6 +353,9 @@ typedef struct MPContext {
     // As video_pts, but is not reset when seeking away. (For the very short
     // period of time until a new frame is decoded and shown.)
     double last_vo_pts;
+    // Frame duration field from demuxer. Only used for duration of the last
+    // video frame.
+    double last_frame_duration;
     // Video PTS, or audio PTS if video has ended.
     double playback_pts;
     // audio stats only

player/video.c

@@ -343,6 +343,7 @@ void reset_video_state(struct MPContext *mpctx)
     mpctx->delay = 0;
     mpctx->time_frame = 0;
     mpctx->video_pts = MP_NOPTS_VALUE;
+    mpctx->last_frame_duration = 0;
     mpctx->num_past_frames = 0;
     mpctx->total_avsync_change = 0;
     mpctx->last_av_difference = 0;
@@ -1369,6 +1370,9 @@ void write_video(struct MPContext *mpctx)
             {
                 MP_VERBOSE(mpctx, "assuming this is an image\n");
                 mpctx->time_frame += opts->image_display_duration;
+            } else if (mpctx->last_frame_duration > 0) {
+                MP_VERBOSE(mpctx, "using demuxer frame duration for last frame\n");
+                mpctx->time_frame += mpctx->last_frame_duration;
             } else {
                 mpctx->time_frame = 0;
             }
@@ -1482,6 +1486,8 @@ void write_video(struct MPContext *mpctx)
 
     mpctx->video_pts = mpctx->next_frames[0]->pts;
     mpctx->last_vo_pts = mpctx->video_pts;
+    mpctx->last_frame_duration =
+        mpctx->next_frames[0]->pkt_duration / mpctx->video_speed;
 
     shift_frames(mpctx);
 

video/decode/vd_lavc.c

@@ -859,6 +859,11 @@ static void decode(struct dec_video *vd, struct demux_packet *packet,
     mpi->pts = mp_pts_from_av(ctx->pic->pts, &ctx->codec_timebase);
     mpi->dts = mp_pts_from_av(ctx->pic->pkt_dts, &ctx->codec_timebase);
 
+#if LIBAVCODEC_VERSION_MICRO >= 100
+    mpi->pkt_duration =
+        mp_pts_from_av(av_frame_get_pkt_duration(ctx->pic), &ctx->codec_timebase);
+#endif
+
     struct mp_image_params params;
     update_image_params(vd, ctx->pic, &params);
     mp_image_set_params(mpi, &params);

video/mp_image.c

@@ -386,6 +386,7 @@ void mp_image_copy_attributes(struct mp_image *dst, struct mp_image *src)
     dst->fields = src->fields;
     dst->pts = src->pts;
     dst->dts = src->dts;
+    dst->pkt_duration = src->pkt_duration;
     dst->params.rotate = src->params.rotate;
     dst->params.stereo_in = src->params.stereo_in;
     dst->params.stereo_out = src->params.stereo_out;

video/mp_image.h

@@ -85,7 +85,7 @@ typedef struct mp_image {
     /* only inside filter chain */
     double pts;
     /* only after decoder */
-    double dts;
+    double dts, pkt_duration;
     /* for private use */
     void* priv;