r11770 - factory smoke ignore slope

[Andarix]

The picture shows what I mean. The smoke on the right building is not above the chimney, but on the chimney.

[prissi]

The smokeoffset for this building is (0,3), i.e. the smoke is put on a tile outside of the building. That tile is flat.

Smoke offsets larger than the factory size should not be used, those could lead even to crashes.

[prissi]

EDIT: Simutrans now corrects wrong offsets on loading time and writes a warning to the log in r11777.