improving path quality on tiled meshes: multiple issues

in the old code, when a new polygon is found a single 3D position is associated with it. This position is used even when arriving at the tile from a different side (at which point the opint is far).
The new code allows multiple nodes for border polygons based on entrance side.
Note also that pidx is only ever assigned from (or used as) a dtNodeIndex, which is an unsigned short, so anything more than 16 bits is pointless anyway.