Fix multi-head placement of overlay (again)

We should be looking at the physical monitor layout, not the current
RFB layout. The latter might not be in sync with the local monitors.
diff --git a/vncviewer/DesktopWindow.cxx b/vncviewer/DesktopWindow.cxx
index a8e17fc..d070b64 100644
--- a/vncviewer/DesktopWindow.cxx
+++ b/vncviewer/DesktopWindow.cxx
@@ -365,20 +365,13 @@
   // Overlay (if active)
   if (overlay) {
     int ox, oy, ow, oh;
-    int sx, sy, sw;
+    int sx, sy, sw, sh;
 
     // Make sure it's properly seen by adjusting it relative to the
     // primary screen rather than the entire window
-    if (fullscreen_active()) {
-      ScreenSet layout;
-
-      layout = cc->cp.screenLayout;
-
-      assert(layout.num_screens() >= 1);
-
-      sx = layout.begin()->dimensions.tl.x;
-      sy = layout.begin()->dimensions.tl.y;
-      sw = layout.begin()->dimensions.width();
+    if (fullscreen_active() && fullScreenAllMonitors) {
+      assert(Fl::screen_count() >= 1);
+      Fl::screen_xywh(sx, sy, sw, sh, 0);
     } else {
       sx = 0;
       sy = 0;