FGroupTopologyDeformer supports deforming a Mesh based on an overlaid FGroupTopology. First the client defines a set of "Handle" elements (Faces/Corners/Edges) using SetActiveHandleX(). The client will provide new vertex positions for the vertices of these elements via the HandleVertexDeformFunc argument to UpdateSolution(). Once the Handle vertices have been updated, the deformer solves for updated vertex positions in the GroupTopology Faces that are adjacent to the handles. This region is referred to as the "ROI" (Region-of-Interest)