Workaround for issue introduced on f2606a9

While the Unity-included version of the C# compiler will happily build the
previous version, the Visual Studio compiler complains that a property on
a struct cannot be accessed until every field has been assigned.

Making the properties public members to cover both cases, since instances
of this struct are ephemeral anyway.
Vector3Pathway.Initialize does not need to be virtual

Any pathway-specific changes or data interpretation should take place at
PrecalculatePathData.