apply a brief blindfold when changing sides in hotseat mp

Fix bug #3865: While the turn dialog used for changing sides is active,
apply a blindfold, which is removed when the dialog is cleared.

changelog

@@ -50,6 +50,8 @@ Version 1.13.0-dev:
    * Fix bug #21257: Lagging animations with skip AI animations and fog/shroud.
    * Improved unicode handling on windows for characters outside the Basic
      Multilingual Plane.
+   * Fix bug #3856: The turn dialog used in hotseat MP play now applies 
+     a blindfold for the duration of the dialog.
 
 Version 1.11.11:
  * Add-ons server:

players_changelog

@@ -21,6 +21,8 @@ Version 1.13.0-dev:
    * Fixed halos glitching through locations that become shrouded after the
      halo is rendered for the first time.
    * OS X user data directory is now ~/Library/Application Support/Wesnoth_1.13
+   * Fix bug #3856: The turn dialog used in hotseat MP play now applies 
+     a blindfold for the duration of the dialog.
 
 
 Version 1.11.11:

src/playsingle_controller.cpp

@@ -741,6 +741,8 @@ void playsingle_controller::before_human_turn(bool save)
 
 void playsingle_controller::show_turn_dialog(){
 	if(preferences::turn_dialog() && (level_result_ == NONE) ) {
+		blindfold b(*resources::screen, true); //apply a blindfold for the duration of this dialog
+		resources::screen->redraw_everything();
 		std::string message = _("It is now $name|’s turn");
 		utils::string_map symbols;
 		symbols["name"] = teams_[player_number_ - 1].current_player();