SDL_offscreenframebuffer.c: fix format '%d' expecting int instead of Uint32

Emitted by Nintendo 3DS's gcc (fix is same as used by ngage)

src/video/offscreen/SDL_offscreenframebuffer.c

@@ -66,7 +66,7 @@ int SDL_OFFSCREEN_UpdateWindowFramebuffer(_THIS, SDL_Window * window, const SDL_
     if (SDL_getenv("SDL_VIDEO_OFFSCREEN_SAVE_FRAMES")) {
         char file[128];
         SDL_snprintf(file, sizeof(file), "SDL_window%d-%8.8d.bmp",
-                     SDL_GetWindowID(window), ++frame_number);
+                     (int)SDL_GetWindowID(window), ++frame_number);
         SDL_SaveBMP(surface, file);
     }
     return 0;