Given the pairing (SourceVertexROI, SourceVertexPositions), compute the symmetry-constrained vertex positions for MirrorVertexROI and store in MirrorVertexPositionsOut. This function assumes that the MirrorVertexROI was computed by calling GetMirrorVertexROI(SourceVertexROI, MirrorVertexROI, bForceSameSizeWithGaps=true), ie all the arrays must be the same length