sub: fix possible deadlock with --no-sub-ass and similar

This is a deadlock caused by a lock order issue: sub/osd.c locks the OSD
first, then the subtitle decoder lock. player/sub.c does the reverse.

Fix this by discussing away the requirement for locking (see below),
which allows us to drop the broken sub lock. sub_get_text() still
acquires and releases the sub decoder lock, but it's not held at the
same time as the OSD lock anymore, so it should be fine.

Originally, the sub lock was acquired because sub_get_text() returns a
pointer to a mutable string. We simply declare that it's ok to call it
unlocked, as long as only 1 thread accesses it, which works out fine in
this case.

player/sub.c

@@ -144,15 +144,10 @@ static void update_subtitle(struct MPContext *mpctx, int order)
 
     // Handle displaying subtitles on terminal; never done for secondary subs
     if (order == 0) {
-        if (!state.render_bitmap_subs || !mpctx->video_out) {
-            sub_lock(dec_sub);
+        if (!state.render_bitmap_subs || !mpctx->video_out)
             set_osd_subtitle(mpctx, sub_get_text(dec_sub, curpts_s));
-            sub_unlock(dec_sub);
-        }
     } else if (order == 1) {
-        sub_lock(dec_sub);
         osd_set_text(mpctx->osd, obj, sub_get_text(dec_sub, curpts_s));
-        sub_unlock(dec_sub);
     }
 }
 

sub/dec_sub.c

@@ -531,6 +531,8 @@ bool sub_has_get_text(struct dec_sub *sub)
 }
 
 // See sub_get_bitmaps() for locking requirements.
+// It can be called unlocked too, but then only 1 thread must call this function
+// at a time (unless exclusive access is guaranteed).
 char *sub_get_text(struct dec_sub *sub, double pts)
 {
     pthread_mutex_lock(&sub->lock);