demux: always update the cache on init

cd59ea8afa removed an arbitrary start
offset added to the timer. However, it turns out that demux secretly
depends on this. When updating cache to actually read bytes from the
stream, there's a diff >= MP_TIME_S_TO_NS(1) check to make it only
update once every second. With the old MP_START_TIME macro, the initial
time value would always be at least 1e10, so this would also be true.
Since we don't have that offset anymore, now the initial time is less
than that so it is not updated and properties like file-size are 0. Just
be sure to always update if the last_speed_query is 0 (i.e. we just
started the player). Fixes #12869.

demux/demux.c

@@ -4131,7 +4131,7 @@ static void update_cache(struct demux_internal *in)
 
     int64_t now = mp_time_ns();
     int64_t diff = now - in->last_speed_query;
-    bool do_update = diff >= MP_TIME_S_TO_NS(1);
+    bool do_update = diff >= MP_TIME_S_TO_NS(1) || !in->last_speed_query;
 
     // Don't lock while querying the stream.
     mp_mutex_unlock(&in->lock);