win32: threads: do not use mdate_perf as the default clock source

It relies on clk.perf.freq.QuadPart which is uninitialized and unknown until
SelectClockSource() is called. So it calls vlc_threads_setup() each time until
vlc_threads_setup() is called with the libvlc instance.

Except vlc_threads_setup(NULL) will crash on the
var_InheritBool(vlc, "high-priority") call. It's broken since
d0c4c5a

Usable candidates at startup are:
* mdate_interrupt which is a counter in 100 ns, not a wall clock
* mdate_tick which is a counter in 1 ms, not a wall clock
* mdate_perf_100ns which is a clock in 100 ns but we don't know if the system
uses that resolution
* mdate_wall which is a clock in 100 ns resolution (slowest)

mdate_wall gives the better results and should provide a safe transition with
mdate_perf/mdate_perf_100ns values.

src/win32/thread.c

@@ -583,8 +583,7 @@ static vlc_tick_t mdate_wall (void)
 
 static vlc_tick_t mdate_default(void)
 {
-    vlc_threads_setup(NULL);
-    return mdate_perf();
+    return mdate_wall();
 }
 
 static vlc_tick_t (*mdate_selected) (void) = mdate_default;