Insert 5 microseconds sleep time each frame.

This shouldn't have any noteable impact on timing (besides the machine
is way too slow for Quake II) and saves a lot of CPU cycles. 100% load
vs. 17% load on my desktop.

src/backends/unix/main.c

@@ -135,6 +135,9 @@ main(int argc, char **argv)
 	/* The mainloop. The legend. */
 	while (1)
 	{
+		struct timespec t = {0, 5000};
+		nanosleep(&t, NULL);
+
 		newtime = Sys_Microseconds();
 		Qcommon_Frame(newtime - oldtime);
 		oldtime = newtime;