Merge branch 'slopes-even-more-fixes' into 'next'

Even more slope-related fixes

What it says on the tin pretty much:

Currently included fixes:
* Pusher specials applied to FOFs did not properly account for slopes applied to the bottom plane; this probably was just a minor slipup that appeared on adding the support in the first place, I take it? (some waterfall in the last section of SUGOI's Fudge Canyon is the only place I can think of offhand where this bug is noticable)

See merge request !114