Change: always assume non-track station ground tile

[glx22]

For station ground sprite, OpenTTD always adds a railtype offset, unless a special trick is used to prevent it (like in example station before this PR).

With this PR, it is now always assumed the station ground sprite is a non-track one unless ground sprite is GROUNDSPRITE_RAIL_X or GROUNDSPRITE_RAIL_Y.

This hides the underlying GRF spec, and should simplify authors' life.

I think railtype offset might still be applied if ground sprite is from a spriteset, but it most likely depends on how the NML is written (sprite: spriteset; vs sprite: spriteset(offset)) and I didn't test, just guessing from the code.