allow half-height elevated way for way_height_clearance=2

[poppo]

Hi!

I heard that way_builder big update will be coming!
I want to add: allow making the half-height elevated way.

You know, when way_height_clearance=2, we can only build elevated way on the double_height("single" height for older than 120).
But I think it should define only "way clearance".
So, when there are no way on the ground, or when the way speed is 0 (=NO VEHICLE USING), it should be allowed to build half-heifht elevated way.

Benefits:

• directly connect from bridge ramp to elevated way(do not need any additional ground)
• more complicated overpass

I'm sorry I do not make the patch for current version, but I paste the pull request for the OTRPatch project (Already merged for OTRPatch).
https://github.com/teamhimeh/simutrans/pull/281
(Several fixes have been made in the current OTRPatch version)
If new way_builder_t is finalaiized, I'll be able to write it.

We need additional window/numimp to define height offset for elevated way, and add the new functions to check above/below tiles clearance.

[prissi]

Current elevated ways do not have the graphics for different heights. That needs to be addressed first, which means new elevated ways anyway.