Feature: LinearGradient color decorator. (#592)

Based on the existing color decorators, create new ones to apply a gradient effect on the DOM.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

examples/component/linear_gradient_gallery.cpp

@@ -0,0 +1,56 @@
+#include <ftxui/component/component_base.hpp>  // for ComponentBase, Component
+#include <ftxui/dom/elements.hpp>  // for operator|, Element, flex, bgcolor, text, vbox, center
+#include <ftxui/dom/linear_gradient.hpp>  // for LinearGradient
+#include <ftxui/screen/color.hpp>         // for Color, Color::Blue, Color::Red
+#include <memory>                         // for __shared_ptr_access, shared_ptr
+#include <string>  // for allocator, operator+, char_traits, string, to_string
+
+#include "ftxui/component/captured_mouse.hpp"  // for ftxui
+#include "ftxui/component/component.hpp"       // for Slider, Renderer, Vertical
+#include "ftxui/component/screen_interactive.hpp"  // for ScreenInteractive
+
+int main(int argc, const char* argv[]) {
+  using namespace ftxui;
+  auto screen = ScreenInteractive::Fullscreen();
+
+  int angle = 180.f;
+  float start = 0.f;
+  float end = 1.f;
+
+  std::string slider_angle_text;
+  std::string slider_start_text;
+  std::string slider_end_text;
+
+  auto slider_angle = Slider(&slider_angle_text, &angle, 0, 360);
+  auto slider_start = Slider(&slider_start_text, &start, 0.f, 1.f);
+  auto slider_end = Slider(&slider_end_text, &end, 0.f, 1.f);
+
+  auto layout = Container::Vertical({
+      slider_angle,
+      slider_start,
+      slider_end,
+  });
+
+  auto renderer = Renderer(layout, [&] {
+    slider_angle_text = "angle = " + std::to_string(angle) + "°";
+    slider_start_text = "start = " + std::to_string(int(start * 100)) + "%";
+    slider_end_text = "end   = " + std::to_string(int(end * 100)) + "%";
+
+    auto background = text("Gradient") | center |
+                      bgcolor(LinearGradient()
+                                  .Angle(angle)
+                                  .Stop(Color::Blue, start)
+                                  .Stop(Color::Red, end));
+    return vbox({
+               background | flex,
+               layout->Render(),
+           }) |
+           flex;
+  });
+
+  screen.Loop(renderer);
+}
+
+// Copyright 2023 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.