dispvm: close only visible windows during DispVM preparation

Closing some invisible window can cause e.g. Firefox crash. Send the
message to visible windows and others should be cleaned up by the
application.

misc/close-window.c

@@ -15,15 +15,26 @@ int close_window(Display *d, XID window) {
 	return XSendEvent(ev.display, ev.window, True, 0, (XEvent *) & ev);
 }
 
+int is_window_visible(Display *d, XID window) {
+    XWindowAttributes xwa;
+
+    if (!XGetWindowAttributes(d, window, &xwa))
+        return 0;
+    return xwa.map_state == IsViewable;
+}
+
 int main(int argc, char **argv) {
 	int i;
 	Display *d;
+	XID w;
 
 	d = XOpenDisplay(NULL);
 	if (!d)
 		exit(1);
 	for (i=1; i<argc; i++) {
-		close_window(d, strtoul(argv[i], NULL, 0));
+        w = strtoul(argv[i], NULL, 0);
+        if (is_window_visible(d, w))
+                close_window(d, w);
 	}
 	XSync(d, False);
 	XCloseDisplay(d);