Handle window closing event to stop SoH (#1556)

* Handle window closing event to stop app

* Address PR comments

libultraship/libultraship/Lib/Fast3D/gfx_sdl2.cpp

@@ -304,6 +304,10 @@ static void gfx_sdl_handle_events(void) {
                     #else
                         SDL_GL_GetDrawableSize(wnd, &window_width, &window_height);
                     #endif
+                } else if (event.window.event == SDL_WINDOWEVENT_CLOSE && event.window.windowID == SDL_GetWindowID(wnd)) {
+                    // We listen specifically for main window close because closing main window
+                    // on macOS does not trigger SDL_Quit.
+                    is_running = false;
                 }
                 break;
             case SDL_DROPFILE: