[r8795] graphical issues with freshly buildt/deleted docks

[Leartin]

In the nightly, when new multi-tile docks are buildt or deleted next to other multi-tile docks, graphical glitches appear. The tiles of the dock that should be behind are in front, and if you delete a dock that is partly covered by another, the covering parts get deleted as well. This does not fix itself with simple scrolling/zooming/mouseover like dirty tiles, but it does if the dock is 'reloaded' (map rotation, load game, move further away and back). Hence I can't provide a game save, but it's easy enough to see with the lighthouse dock in pak64.

I assume this is a side-effect of a recent patch: https://forum.simutrans.com/index.php/topic,19097.0.html

[Dwachs]

should be fixed in r8816