Building tunnels across sloped tunnels does not fail

[ceeac]

This is a follow-up of https://forum.simutrans.com/index.php?msg=211051

Building a tunnel across the top of a sloped tunnel tile succeeds (tool_build_tunnel does not return an error) but no tunnel tiles are built.
Building a tunnel down a double slope will build tunnel tiles two levels below the intended height.

[prissi]

r11782 implements this check after fixing an error (no 3D coordinate check in route search if exit tile was on map ground, i.e. a portal.) That error must be in for at least 20 years.