Fold the given node

Fold the given node. Given a node N, collapses the representative vertices of the children of N to the representative vertex of N. This removes some triangles from the scene (namely, N->subtris). N is assumed to lie just above the active boundary before the foldNode() operation, and lies on it afterwards.Requires: All of N's child nodes must be labeled Boundary. If this is not the case, use vdsFoldSubtree().

Implementation note: The active boundary is maintained as a circular linked list which always starts at the root, goes to the leftmost node labeled * Boundary, threads through the active boundary of the vertex tree to the rightmost Boundary node, and returns to the root.

**Parameters:****node**- The node to be folded.

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