[rr7780] bridge pillars suffer from terraforming

[gauthier]

Simutrans nightly rr7780 (happens with 120.1.3 too), pak128 2.6, Win7.

Build a bridge with pillars, use terraforming tool (artificial slopes) to lower tiles under the bridge. Pillars go down with the tiles but don't grow up. (No it's not a glitch in the bridge's graphics, I checked it, and it happens with all bridges with pillars.)