Mario: add basic main menu to editor, separate scenes from main game loop

mario/global_vars.cpp

@@ -10,3 +10,4 @@ std::shared_ptr<SDLPP::Texture> g_translucent_mod_texture{};
 std::shared_ptr<SDLPP::Texture> g_translucent_enemies_texture;
 std::shared_ptr<SDLPP::Scene> g_playground{};
 std::shared_ptr<SDLPP::FontConfiguration> g_text_config{};
+bool g_quit = false;