gui: Handle SDL_WINDOWEVENT_SIZE_CHANGED

Instead of SDL_WINDOWEVENT_RESIZED.

The documentation is a bit unclear, but SDL_WINDOWEVENT_RESIZED seems
not to trigger under certain circumstances, specifically when changing
resolution via SDL_SetWindowSize(); SDL_WINDOWEVENT_SIZE_CHANGED
meanwhile is implied to trigger any time the window size changes
whether it's from an API call, or the operating system, or a user
action.

<https://wiki.libsdl.org/SDL2/SDL_WindowEventID>

We need to translate at least one of these events into a GUI2 event in
order for dialogs to be properly re-laid out after the window size
changes, but we should not handle both because that could result in
unnecessary work recalculating all dialog layouts.

Closes #7436.

changelog.md

@@ -13,6 +13,8 @@
  ### User interface
    * Fixed main menu Language button not being refreshed after switching languages
      without relaunching the game (issue #7437).
+   * Fixed changing game resolution in Preferences not refreshing the user interface
+     as expected (issue #7436).
  ### WML Engine
    * Add support for distinct sub-achievements.
  ### Miscellaneous and Bug Fixes

src/gui/core/event/handler.cpp

@@ -441,7 +441,10 @@ void sdl_event_handler::handle_event(const SDL_Event& event)
 
 		case SDL_WINDOWEVENT:
 			switch(event.window.event) {
-				case SDL_WINDOWEVENT_RESIZED:
+				// Always precedes SDL_WINDOWEVENT_RESIZED, but the latter does not always
+				// happen; in particular when we change the game resolution via
+				// SDL_SetWindowSize() <https://github.com/wesnoth/wesnoth/issues/7436>
+				case SDL_WINDOWEVENT_SIZE_CHANGED:
 					video_resize(video::game_canvas_size());
 					break;