corpseslide: rewrote sliding code (hull creation and such)
use Andrew's Monotone Chain algo for convex hull creation: it is smaller
than Gift Wrapping, and has no problems with duplicate/collinear points.
also, clip resulting hull instead of each subsector. this is much faster
too. and don't bother checking mobj origin: "stability checker" will do
it for us anyway.
TODO: got rid of origin checks completely. to check for slopes, loop over
all touched sectors, find the proper region, and check its floor.
FossilOrigin-Name: e6e91e15d013b3f5ecbde866ce032ef48481f0a1e4d5ebc443fc71238c730cc1