Callback: Custom shape check

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Custom shape check (2F)

This callback is called when TTD checks if a given tile is suitable for the industry tile. Since the tile isn't built yet, you can use variables 41, 42, 43, and 60 only, and you can't access any industry variables, either. Since OpenTTD r19744 however, you can access the random bits of the industry (it would get after construction) via industry variable 5F resp. RandomAction2 type 83. Var. 60 is the most useful here, since you can check the slope of the tile, or even the nearby tiles. This callback is the only way to allow your tile to be built on steep slopes. The callback should return zero if the tile isn't suitable, or any other value if it is suitable.

Since TTDPatch 2.6 r1728, the lowest byte of variable 18 contains the layout number selected for the industry. The other bits of variable 18 remain undefined for now.

Since OpenTTD r20942, the second lowest byte of variable 18 contains the trigger event of the industry construction. The values are the same as the lowest byte of variable 18 of callback 28.

Important: The meaning of the returned value will change in GRF version 7; it will work the same way as callback 28. Also, since TTDPatch r1755, you can use the text reference stack for your error messages, similarly to callback 3A. The only difference is that only 4 registers are copied instead of 6; see callback 28 for details.

PLEASE NOTE: When this callback is called, TTD is just checking, and hasn't placed any industry tiles yet. Therefore, bit 8 (tile belongs to the same industry as current) of variable 60 will always be clear, even for the current tile.