Implement MH's version of Sys_DoubleTime for windows.

From http://forums.inside3d.com/viewtopic.php?p=36239

libs/util/sys.c

@@ -289,6 +289,7 @@ Sys_DoubleTime (void)
 {
 	static qboolean first = true;
 #ifdef _WIN32
+# if 0
 	static DWORD starttime;
 	DWORD       now;
 
@@ -307,6 +308,51 @@ Sys_DoubleTime (void)
 		return 0.0;
 
 	return (now - starttime) / 1000.0;
+# else
+	// MH's solution combining timeGetTime for stability and
+	// QueryPerformanceCounter for precision.
+	static __int64 qpcfreq = 0;
+	static __int64 currqpccount = 0;
+	static __int64 lastqpccount = 0;
+	static double qpcfudge = 0;
+	DWORD currtime = 0;
+	static DWORD lasttime = 0;
+	static DWORD starttime = 0;
+
+	if (first) {
+		timeBeginPeriod (1);
+		starttime = lasttime = timeGetTime ();
+		QueryPerformanceFrequency ((LARGE_INTEGER *) &qpcfreq);
+		QueryPerformanceCounter ((LARGE_INTEGER *) &lastqpccount);
+		first = false;
+		return 0;
+	}
+
+	// get the current time from both counters
+	currtime = timeGetTime ();
+    QueryPerformanceCounter ((LARGE_INTEGER *) &currqpccount);
+
+	if (currtime != lasttime)  {
+		// requery the frequency in case it changes (which it can on multicore
+		// machines)
+		QueryPerformanceFrequency ((LARGE_INTEGER *) &qpcfreq);
+
+		// store back times and calc a fudge factor as timeGetTime can
+		// overshoot on a sub-millisecond scale
+		qpcfudge = (((double) (currqpccount - lastqpccount)
+					/ (double) qpcfreq))
+				- ((double) (currtime - lasttime) * 0.001);
+		lastqpccount = currqpccount;
+		lasttime = currtime;
+	} else {
+		qpcfudge = 0;
+	}
+
+	// the final time is the base from timeGetTime plus an addition from QPC
+	return ((double) (currtime - starttime) * 0.001)
+		+ ((double) (currqpccount - lastqpccount) / (double) qpcfreq)
+		+ qpcfudge;
+# endif
 #else
 	struct timeval tp;
 	struct timezone tzp;