Fix #1736: on GNU/Linux, a hotkey can trigger multiple commands

SDL_TEXTINPUT events aren't generated on Windows when Alt is held down.
Let's ensure that the behavior is the same on all platforms.

(cherry-picked from commit c42401a8de)

src/events.cpp

@@ -477,6 +477,15 @@ void pump()
 		last_resize_event_used = true;
 	}
 
+	if(SDL_GetModState() & KMOD_ALT) {
+		// Remove text input events, to match the behavior on Windows
+		// See https://github.com/wesnoth/wesnoth/issues/1736
+		events.erase(
+			std::remove_if(events.begin(), events.end(), [](const SDL_Event& e) { return e.type == SDL_TEXTINPUT; }),
+			events.end()
+		);
+	}
+
 	// Move all draw events to the end of the queue
 	auto first_draw_event = std::stable_partition(events.begin(), events.end(),
 		[](const SDL_Event& e) { return e.type != DRAW_EVENT; }