[AD] Added yield_timeslice to vsync

[Jon Rafkind <workmin@xxxxxxxxxx>]

Attached is a very small patch( one line ) that adds yield_timeslice to 
the vsync call for the xwindows driver. I suppose yielding in the loop 
while waiting for the vsync to finish is a bit dangerous, but I didnt 
encounter any problems with it. I guess yield_timeslice could be added 
to the other drivers, but I wanted to wait and see if it is ok before I 
go ahead and do that.
later--,
jon

--- xwin2.c	2004-04-03 21:48:40.000000000 -0500
+++ xwin.c	2004-04-03 21:47:21.000000000 -0500
@@ -1941,6 +1941,7 @@
       int prev = retrace_count;
 
       do {
+      	yield_timeslice();
       } while (retrace_count == prev);
    }
 }