Merge branch 'missiles_n_slopes' into 'next'

Missiles 'n slopes

Fixed that thing where missiles like sliding up slopes for some reason.

This isn't a 1:1 fix compared to non-slopes - missiles still like stepping up over the borders of sloped sectors and will continue going as long as they never touch that ground again, whereas they can never step up on flat ones - but this fixes the most egregrious issue.

See merge request !181