Increase max size of single player world icon.png from 64x to 256x.

Right now world icons are based on the first things you see when you open a world when no icon has been given to the world. In the folder of the world is a file called icon.png that is the icon of the world. This can be replaced by any 64x64 picture.

It would be a nice small change that this icon could also be the size of 128x128 or 256x256 (or even 512x512（づ￣3￣）づ), but I would also understand that since it is not common to use custom picture for icons of worlds that this change does not really matter. Nevertheless, since this is a small adjustment I felt the need to ask it here.