make adaptive menu fps optional ('display.adaptive-menu-fps' flag)

src/engine/Mainloop.cpp

@@ -18,6 +18,7 @@ Mainloop::Mainloop(Engine& engine) : engine(engine) {
 void Mainloop::run() {
     auto& time = engine.getTime();
     auto& window = engine.getWindow();
+    auto& settings = engine.getSettings();
 
     engine.setLevelConsumer([this](auto level, int64_t localPlayer) {
         if (level == nullptr) {
@@ -46,6 +47,7 @@ void Mainloop::run() {
         }
         engine.postUpdate();
         engine.nextFrame(
+            settings.display.adaptiveFpsInMenu.get() &&
             dynamic_cast<const MenuScreen*>(engine.getScreen().get()) != nullptr
         );
     }

src/io/settings_io.cpp

@@ -51,6 +51,7 @@ SettingsHandler::SettingsHandler(EngineSettings& settings) {
     builder.add("framerate", &settings.display.framerate);
     builder.add("limit-fps-iconified", &settings.display.limitFpsIconified);
     builder.add("window-mode", &settings.display.windowMode);
+    builder.add("adaptive-menu-fps", &settings.display.adaptiveFpsInMenu);
 
     builder.section("camera");
     builder.add("sensitivity", &settings.camera.sensitivity);

src/settings.hpp

@@ -31,6 +31,8 @@ struct DisplaySettings {
     IntegerSetting framerate {-1, -1, 120};
     /// @brief Limit framerate when window is iconified
     FlagSetting limitFpsIconified {false};
+    /// @brief Adaptive framerate in menu (experimental)
+    FlagSetting adaptiveFpsInMenu {false};
 };
 
 struct ChunksSettings {