Add a missing break;

Whoops. Now I wonder why the compile didn't fail for me locally, even
though I *should* have -Werror=implicit-fallthrough enabled...

desktop_version/src/Graphics.cpp

@@ -1509,6 +1509,7 @@ void Graphics::processfade(void)
         {
             fademode = FADE_NONE;
         }
+        break;
     case FADE_NONE:
     case FADE_FULLY_BLACK:
         break;