Reset dead keys when the SDL window loses focus, so dead keys pressed in SDL applications don't affect text input into other applications.

src/video/windows/SDL_windowsevents.c

@@ -451,6 +451,7 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 
                 if (SDL_GetKeyboardFocus() == data->window) {
                     SDL_SetKeyboardFocus(NULL);
+                    WIN_ResetDeadKeys();
                 }
 
                 ClipCursor(NULL);