sub/osd: add log if osd rendering takes too long time

Ideally it should be fast, it is on VO thread and steals precious frame
time budget. Looks like this is one of the main reasons why
display-resample on higher refresh rate displays is too slow.

sub/osd.c

@@ -333,6 +333,8 @@ struct sub_bitmap_list *osd_render(struct osd_state *osd, struct mp_osd_res res,
 {
     mp_mutex_lock(&osd->lock);
 
+    int64_t start_time = mp_time_ns();
+
     struct sub_bitmap_list *list = talloc_zero(NULL, struct sub_bitmap_list);
     list->change_id = 1;
     list->w = res.w;
@@ -388,6 +390,11 @@ struct sub_bitmap_list *osd_render(struct osd_state *osd, struct mp_osd_res res,
     if (!(draw_flags & OSD_DRAW_SUB_FILTER))
         osd->want_redraw_notification = false;
 
+    double elapsed = MP_TIME_NS_TO_MS(mp_time_ns() - start_time);
+    bool slow = elapsed > 5;
+    mp_msg(osd->log, slow ? MSGL_DEBUG : MSGL_TRACE, "Spent %.3f ms in %s%s\n",
+           elapsed, __func__, slow ? " (slow!)" : "");
+
     mp_mutex_unlock(&osd->lock);
     return list;
 }