courtesy of Nev3r
Ported from SRB2
STJr/SRB2@5401e956
and follow up commits, there was no attached pr sadly
Fixes join desynchs on maps with "wavy floors" like in Opulence since dynamic slopes use thinkers now thus can be synched which prevents things from falling through them after loading the netsave
Tested with MOBJCONSISTENCY to ensure everythink synchs correctly
Attached is a test map made by Patafoin which was very helpful
TestSlopedynamic.wad
