New winter images for road/rail crossings

[ceeac]

Tried to use GIMP. Results are acceptable for me as a non-artist.
https://github.com/simutrans/pak128/pull/34

[Yona-TYT]

Tried to use GIMP. Results are acceptable for me as a non-artist.
https://github.com/simutrans/pak128/pull/34

looks good to me, but I'm not a pak128 maintainer so I can't merge without discussing it first.

[prissi]

Certainly better than anything beforehand. I have also updated the steam and smoke from dither to real transparency with the help of Leartin.

compat.tab is useless for crossings; these will be recreated based on speed on loading; for old games, it will fatally fail if there is no legal crossing. The name is never saved.

The current pak128 has also a very much extended menu structure, more like the new pak64, so all the various options are now in a graphical menu (toolbar), inspired from pak192comic with lots of previously misssing tools.