Add setRepaintWhenIdle()

Add a function which will register a (Idle-) work procedure to do the
redraw when there are no more other events pending.

If the window size has changed after the last redraw, clear the
window to clean up wrongly scaled relicts.

xwin.c

@@ -354,6 +354,31 @@ static Visual* vis;
 static Window win;
 static GC gc;
 static XtAppContext app_con;
+static int workerRegistered=0;
+static int last_repaint_width=-1;
+static int last_repaint_height=-1;
+
+Boolean doIdleRepaint(XtPointer data) {
+    (void)data;
+    XWindowAttributes xwa;
+
+    XGetWindowAttributes(dpy, win, &xwa);
+    if (xwa.width != last_repaint_width || xwa.height != last_repaint_height) {
+        XClearWindow(dpy, win);
+    }
+    repaint(xwa.width, xwa.height);
+    last_repaint_width=xwa.width;
+    last_repaint_height=xwa.height;
+    workerRegistered=0;
+    return True;
+}
+
+static void setRepaintWhenIdle() {
+        if (!workerRegistered) {
+            (void)XtAppAddWorkProc(app_con,doIdleRepaint,NULL);
+            workerRegistered=1;
+        }
+}
 
 static void
 a_savesvg(Widget w, XEvent* e, String* params, Cardinal* num_params)