This reverts commit 75633bde.

Polyobj_rotate and related: change "origin" to vector2_t since all it needs is x/y and not any of the new stuff for vertex_t

Effect 6 flag allows cutting of cyan pixels on FOF and PolyObject planes (even if they're translucent)

T_PolyObjWaypoint: We can find waypoints in constant time now, so no need to store the waypoint mobj in the thinker anymore

Replace most P_FindSpecialLineFromTag() cases with Tag_FindLineSpecial(), which settle with the first found result.

- Fixed loss of precision in rotate carry causing objects to slide off
- Adjusted player carrying logic to make platforms less slippery
- Finally obsoleted the player-specific rotate hack now that I found the
  actual problem :]

added a "target" pointer to polywaypoint_t, so the polyobj waypoint thinker doesn't have to re-find the next waypoint every tic