Fix focus vs flexbox interaction. (#405)

- Fix focus in flexbox. This required resetting the focus state at the
  beginning of the ComputeRequirement(), because it can now run several
  times.

  This resolves:https://github.com/ArthurSonzogni/FTXUI/issues/399

- Add Box::Union.

- Add a preliminary implementation of forwarding selected_box from
  within the flexbox.

src/ftxui/screen/box.cpp

@@ -15,6 +15,18 @@ Box Box::Intersection(Box a, Box b) {
   };
 }
 
+/// @return the smallest Box containing both |a| and |b|.
+/// @ingroup screen
+// static
+Box Box::Union(Box a, Box b) {
+  return Box{
+      std::min(a.x_min, b.x_min),
+      std::max(a.x_max, b.x_max),
+      std::min(a.y_min, b.y_min),
+      std::max(a.y_max, b.y_max),
+  };
+}
+
 /// @return whether (x,y) is contained inside the box.
 /// @ingroup screen
 bool Box::Contain(int x, int y) const {

src/ftxui/screen/terminal.cpp

@@ -20,8 +20,8 @@ namespace ftxui {
 
 namespace {
 
-bool g_cached = false;
-Terminal::Color g_cached_supported_color;
+bool g_cached = false;                     // NOLINT
+Terminal::Color g_cached_supported_color;  // NOLINT
 
 Dimensions& FallbackSize() {
 #if defined(__EMSCRIPTEN__)