Add TODO comment for cvstat.

When originally created, it was intended to be accessed only by a
single thread, and therefore not need mutex protection, allowing
less code to run with vstatlock held.  Unfortunately, this is not
how it turned out, so cvstat should either be removed, or it should
actually be cleaned up to work as intended.

src/conio/sdl_con.c

@@ -65,6 +65,17 @@ pthread_mutex_t sdl_keylock;
 sem_t sdl_key_pending;
 static unsigned int sdl_pending_mousekeys=0;
 
+/*
+ * TODO:
+ * The intent of this was a copy of current vstat that
+ * does not require vstatlock to be held.
+ *
+ * Unfortunately, this is accessed from multiple threads
+ * currently (at least main and video events), so it
+ * requires *a* lock, and vstatlock is used for that purpose.
+ * This should either have its own lock, or be used *only*
+ * from a single thread.
+ */
 static struct video_stats cvstat;
 
 struct sdl_keyvals {