The MoveSubtree() function moves trees from lower down in the KDTree
up. The existing depth-first traversal recursive algorithm in this
function worked okay when descending the tree, but when moving back up
the tree, elements in the array would be clobbered before they were
moved to their correct destination. This would lead to some views
being incorrectly deleted and moved into the wrong location when a
level of the KDTree was being collapsed.

Change to a breadth-first algorithm so that elements in the tree are
always copied from lower to higher positions in the tree. This
resolves the incorrect subtree movement.