Backends: SDL3: fixed typo leading to PlatformHandleRaw not being set leading to SHOWNA path not working for multi-viewports.

backends/imgui_impl_sdl3.cpp

@@ -386,7 +386,7 @@ static void ImGui_ImplSDL3_SetupPlatformHandles(ImGuiViewport* viewport, SDL_Win
 {
     viewport->PlatformHandle = window;
     viewport->PlatformHandleRaw = nullptr;
-#if defined(__WIN32__) && !defined(__WINRT__)
+#if defined(_WIN32) && !defined(__WINRT__)
     viewport->PlatformHandleRaw = (HWND)SDL_GetPointerProperty(SDL_GetWindowProperties(window), SDL_PROP_WINDOW_WIN32_HWND_POINTER, nullptr);
 #elif defined(__APPLE__) && defined(SDL_VIDEO_DRIVER_COCOA)
     viewport->PlatformHandleRaw = SDL_GetPointerProperty(SDL_GetWindowProperties(window), SDL_PROP_WINDOW_COCOA_WINDOW_POINTER, nullptr);