Support partial window

[bwikbs]

Preparation work for #94

• Now it is possible to create a partial app instead of a full screen as follows:

  Ecore_Wl2_Display* ecore_wl2_display_ = nullptr;
  ecore_wl2_init();
  ecore_wl2_display_ = ecore_wl2_display_connect(nullptr);
  ecore_wl2_sync();
  Ecore_Wl2_Window* ecore_wl2_window_ =
      ecore_wl2_window_new(ecore_wl2_display_, nullptr, 100, 100, 500, 600);
  ecore_wl2_window_type_set(ecore_wl2_window_,
                            ECORE_WL2_WINDOW_TYPE_TOPLEVEL);
  ecore_wl2_window_alpha_set(ecore_wl2_window_, false);
  ecore_wl2_window_position_set(ecore_wl2_window_, 0, 0);
  ecore_wl2_window_aux_hint_add(ecore_wl2_window_, 0,
                                "wm.policy.win.user.geometry", "1");
  
  FlutterDesktopEngineProperties engine_prop = {};
  engine_prop.assets_path = assets_path.c_str();
  engine_prop.icu_data_path = icu_data_path.c_str();
  engine_prop.aot_library_path = aot_lib_path.c_str();
  engine_prop.switches = switches.data();
  engine_prop.switches_count = switches.size();
  engine_prop.custom_win = ecore_wl2_window_;