Fixes GH-1295
Non-standard sized portals exacerbate a flaw in the vanilla
portal teleportation adjustment logic.
As a result, an entity can end up slightly inside of the surrounding
portal blocks. In vanilla, this issue is minor and you are adjusted out
as if it never happened. In CraftBukkit and derivatives, the
anti-suffocation behavior activates and players end up teleported on top
of their portals.
This improves the offset so as to keep the issue from ever occurring in
the first place.
Special thanks to CarpetMod who appears to have had this fixed for some
time, and has licensed their code such that we can use it as needed.
Zach Brown