Restore doing nothing for calls to VGLEnd() after the first. I broke this

in r346631.  VGLEnd() clears some state variables as it restores state,
but not all of them, so it still needs to clear a single state variable
to indicate that it has completed.  Put this clearing back where it was
(at the start instead of the end) to avoid moving bugs in the signal
handling.

lib/libvgl/main.c

@@ -73,6 +73,7 @@ struct vt_mode smode;
 
   if (!VGLInitDone)
     return;
+  VGLInitDone = 0;
   signal(SIGUSR1, SIG_IGN);
   signal(SIGUSR2, SIG_IGN);
   VGLSwitchPending = 0;