opengl: drop less frames when clip and display have similar fps

adds 1 vsync interval "slack" before deciding to drop the first frame. it should
help on cases of timing jitter (sleep duration, container timestamps, compositor
vsync timing, etc). once the drop threshold has been crossed, it will keep
dropping until perfect timing alignment. this prevents crossing the drop
threshold back and forth repeatedly and therefore more resilient to frame drops

video/out/vo.c

@@ -605,7 +605,15 @@ static bool render_frame(struct vo *vo)
     if (!in->hasframe_rendered)
         duration = -1; // disable framedrop
 
-    in->dropped_frame = duration >= 0 && end_time < next_vsync;
+    // if the clip and display have similar/identical fps, it's possible that
+    // we'll be very slightly late frequently due to timing jitter, or if the
+    // clip/container timestamps are not very accurate.
+    // so if we dropped the previous frame, keep dropping until we're aligned
+    // perfectly, else, allow some slack (1 vsync) to let it settle into a rhythm.
+    in->dropped_frame = duration >= 0 &&
+                            ((in->dropped_frame && end_time < next_vsync) ||
+                            (end_time < prev_vsync)); // hard threshold - 1 vsync late
+
     in->dropped_frame &= !(vo->driver->caps & VO_CAP_FRAMEDROP);
     in->dropped_frame &= (vo->global->opts->frame_dropping & 1);
     // Even if we're hopelessly behind, rather degrade to 10 FPS playback,