Prevent accidental skip cutscene at loading screen

`os_ignore_events()` deferred events instead of ignoring them. The
player could accidentally skip a cutscene at the loading screen and
not see any indication there was a cutscene.

This only removes deferred skip-cutscene key events, except ESC, right
before the cutscene plays. This makes it less likely for a player to
accidentally skip the cutscene.

This also renames `buffered_events` to `deferred_events` and
`os_ignore_events()` to `os_defer_events_on_load_screen()`, which more
accurately describes the behavior.

Fixes #4419.

Author: daftmugi

code/cutscene/movie.cpp

@@ -51,6 +51,7 @@ void processEvents()
 {
 	io::mouse::CursorManager::get()->showCursor(false);
 
+	os_remove_deferred_cutscene_key_events();
 	os_poll();
 }
 

code/osapi/osapi.cpp

@@ -300,7 +300,7 @@ static char			szWinTitle[128];
 static char			szWinClass[128];
 static int			Os_inited = 0;
 
-static SCP_vector<SDL_Event> buffered_events;
+static SCP_vector<SDL_Event> deferred_events;
 
 int Os_debugger_running = 0;
 
@@ -713,10 +713,11 @@ namespace os
 	}
 }
 
-void os_ignore_events() {
+void os_defer_events_on_load_screen() {
 	SDL_Event event;
 	while (SDL_PollEvent(&event)) {
-		// ignore event
+		// Add event to be handled later
+		deferred_events.push_back(event);
 	}
 }
 
@@ -744,14 +745,28 @@ static void handle_sdl_event(const SDL_Event& event) {
 	}
 }
 
+void os_remove_deferred_cutscene_key_events() {
+	deferred_events.erase(
+		std::remove_if(deferred_events.begin(), deferred_events.end(), [](const SDL_Event &event)
+		{
+			return (event.type == SDL_KEYUP) && (
+				event.key.keysym.sym == SDLK_KP_ENTER ||
+				event.key.keysym.sym == SDLK_RETURN ||
+				event.key.keysym.sym == SDLK_SPACE
+			);
+		}),
+		deferred_events.end()
+	);
+}
+
 void os_poll()
 {
-	// Replay the buffered events
-	auto end = buffered_events.end();
-	for (auto it = buffered_events.begin(); it != end; ++it) {
+	// Replay the deferred events
+	auto end = deferred_events.end();
+	for (auto it = deferred_events.begin(); it != end; ++it) {
 		handle_sdl_event(*it);
 	}
-	buffered_events.clear();
+	deferred_events.clear();
 
 	SDL_Event event;
 

code/osapi/osapi.h

@@ -64,9 +64,12 @@ int os_foreground();
 // process management --------------------------------------------------------------
 
 /**
- * @brief Removes all pending events and ignores them
+ * @brief Defers all pending events to be handled later
  */
-void os_ignore_events();
+void os_defer_events_on_load_screen();
+
+// removes key events that may unintentionally skip the cutscene
+void os_remove_deferred_cutscene_key_events();
 
 // call to process windows messages. only does something in non THREADED mode
 void os_poll();

freespace2/freespace.cpp

@@ -1192,7 +1192,7 @@ void game_loading_callback(int count)
 
 	Script_system.RemHookVar("Progress");
 
-	os_ignore_events();
+	os_defer_events_on_load_screen();
 
 	if (do_flip)
 		gr_flip();