Add SDL_Quit calls back in

These got lost in the migration to Meson; they used to be called
with atexit, but this caused more problems than it should have.
Anyway, it's fine to call these only when we're exiting normally,
since otherwise we have bigger problems than not quitting SDL.

src/PowderToyFontEditor.cpp

@@ -471,5 +471,6 @@ int main(int argc, char * argv[])
 	ui::Engine::Ref().CloseWindow();
 	delete gameController;
 	delete ui::Engine::Ref().g;
+	SDL_Quit();
 	return 0;
 }

src/PowderToySDL.cpp

@@ -963,5 +963,6 @@ int main(int argc, char * argv[])
 	delete gameController;
 	delete ui::Engine::Ref().g;
 	Client::Ref().Shutdown();
+	SDL_Quit();
 	return 0;
 }