Add the modal dialog example.

New component: clear_under.

src/ftxui/dom/clear_under.cpp

@@ -0,0 +1,35 @@
+#include "ftxui/dom/elements.hpp"
+#include "ftxui/dom/node_decorator.hpp"
+
+namespace ftxui {
+
+using ftxui::Screen;
+
+class ClearUnder : public NodeDecorator {
+ public:
+  ClearUnder(Elements children) : NodeDecorator(std::move(children)) {}
+  ~ClearUnder() override {}
+
+  void Render(Screen& screen) override {
+    for (int y = box_.y_min; y <= box_.y_max; ++y) {
+      for (int x = box_.x_min; x <= box_.x_max; ++x) {
+        screen.PixelAt(x, y) = Pixel();
+      }
+    }
+    Node::Render(screen);
+  }
+};
+
+/// @brief Before drawing |child|, clear the pixels below. This is useful in
+//         combinaison with dbox.
+/// @see ftxui::dbox
+/// @ingroup dom
+Element clear_under(Element child) {
+  return std::make_shared<ClearUnder>(unpack(std::move(child)));
+}
+
+}  // namespace ftxui
+
+// Copyright 2020 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.