player/video: only use MIN_PAST_FRAMES check if speed was changed

Otherwise seeking at very high speeds could have some desync since it
naturally desyncs much faster at higher speeds. More clearly restrict
this behavior to the speed change case.

Fixes e3af545

player/core.h

@@ -406,6 +406,8 @@ typedef struct MPContext {
     /* Heuristic for potentially redrawing subs. */
     bool redraw_subs;
 
+    bool speed_changed;
+
     bool paused;            // internal pause state
     bool playback_active;   // not paused, restarting, loading, unloading
     bool in_playloop;

player/video.c

@@ -106,6 +106,7 @@ void reset_av_state(struct MPContext *mpctx)
     mpctx->logged_async_diff = -1;
     mpctx->num_past_frames = 0;
     mpctx->total_avsync_change = 0;
+    mpctx->speed_changed = true;
 }
 
 void reset_video_state(struct MPContext *mpctx)
@@ -606,13 +607,14 @@ static void update_avsync_before_frame(struct MPContext *mpctx)
     if (mpctx->video_status < STATUS_READY) {
         mpctx->time_frame = 0;
     } else if (mpctx->display_sync_active || vo->opts->video_sync == VS_NONE ||
-               mpctx->num_past_frames <= MIN_PAST_FRAMES)
+               (mpctx->num_past_frames <= MIN_PAST_FRAMES && mpctx->speed_changed))
     {
         // don't touch the timing
     } else if (mpctx->audio_status == STATUS_PLAYING &&
                mpctx->video_status == STATUS_PLAYING &&
                !ao_untimed(mpctx->ao))
     {
+        mpctx->speed_changed = false;
         double buffered_audio = ao_get_delay(mpctx->ao);
 
         double predicted = mpctx->delay / mpctx->video_speed +
@@ -846,8 +848,9 @@ static void handle_display_sync_frame(struct MPContext *mpctx,
     if (resample && using_spdif_passthrough(mpctx))
         return;
 
-    if (mpctx->num_past_frames <= MIN_PAST_FRAMES)
+    if (mpctx->num_past_frames <= MIN_PAST_FRAMES && mpctx->speed_changed)
         return;
+    mpctx->speed_changed = false;
 
     double vsync = vo_get_vsync_interval(vo) / 1e9;
     if (vsync <= 0)