The geometry tree nodes contain a position element (pos) that identifies
the spatial position of the node.  At internal nodes this node position
is the partial position of a point and may either be represented using
the magnitude of the decoded points, or the magnitude of the current
depth.

The first form equates to:
 nextPos = curPos | (xyz << currentNodeSize)

The second:
 nextPos = (curPos << 1) | xyz

Where xyz represents a position bit from the coded occupancy.

In order to use the first form to determine neighbour relationships,
curPos must be shifted down to represent position within the current
depth.  This repeated shifting, along with the introduction of
simultaneous quantised and unquantised positions becomes increasingly
difficult to follow when adding non-cubic nodes (via OtQtBt).

This commit switches the representation to the second form above,
eliminating various shifted forms and duplicate state.  A decoded
positions is inverse quantised when reaching a leaf node.