fix fullscreen clients not resized on X display resolution change

patch provided by Bert Münnich <ber.t_AT_posteo.de>, thanks!

dwm.c

@@ -565,6 +565,7 @@ void
 configurenotify(XEvent *e)
 {
 	Monitor *m;
+	Client *c;
 	XConfigureEvent *ev = &e->xconfigure;
 	int dirty;
 
@@ -576,8 +577,12 @@ configurenotify(XEvent *e)
 		if (updategeom() || dirty) {
 			drw_resize(drw, sw, bh);
 			updatebars();
-			for (m = mons; m; m = m->next)
+			for (m = mons; m; m = m->next) {
+				for (c = m->clients; c; c = c->next)
+					if (c->isfullscreen)
+						resizeclient(c, m->mx, m->my, m->mw, m->mh);
 				XMoveResizeWindow(dpy, m->barwin, m->wx, m->by, m->ww, bh);
+			}
 			focus(NULL);
 			arrange(NULL);
 		}