Draft: Use Thinkers for Dynamic Slopes to fix desynchs

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

Funny Opulence image from testing