Mouse support. Fix & verify Webassembly support.

There was some undefined behavior to be fixed in the terminal input
parser.

The behavior of flush seems to have change. The fix was to invert '\0'
and std::flush.

src/ftxui/component/screen_interactive.cpp

@@ -40,7 +40,7 @@ namespace {
 
 void Flush() {
   // Emscripten doesn't implement flush. We interpret zero as flush.
-  std::cout << std::flush << (char)0;
+  std::cout << '\0' << std::flush;
 }
 
 constexpr int timeout_milliseconds = 20;
@@ -353,6 +353,8 @@ void ScreenInteractive::Loop(Component* component) {
       DECMode::kMouseSgrExtMode,
   });
 
+  flush();
+
   auto event_listener =
       std::thread(&EventListener, &quit_, event_receiver_->MakeSender());