Remove "z" from "vertex_t"

This removes vertex_t's z variable, as it is unnecessary. The only occurrence across the entire codebase for this variable was a hack used by the rope hang snapping behavior; this now uses vector3_t for the necessary calculations. P_ClosestPointOnLine3D() was also adapted to make use of vector3_t. (A similar function, FV3_ClosestPointOnLine(), is meant to have the same functionality, but the math there fails, possibly due to fixed point overflows, and therefore P_ClosestPointOnLine3D() was kept and adapted for this purpose.)