Fix inconsistent sector special detection on dynslopes

This is a port of this fix I made for SRB2 Classic: https://codeberg.org/srb2classic/srb2classic/pulls/52

This fixes the inconsistent/non-existent application of specials on dynslopes to players, it seems to be an issue with thinker list order causing it to update after the player does meaning ran very strangely.

Not sure if anything relies on this broken behavior but its definitely a regression as it functions fine as expected in 2.1 and Kart (which is 2.1 based).