In cases like assigning UVs or Normals to a QuadGridPatch, we want to assign a unique element to each of the vertices in the grid, and then generate "element triangles" that index into those new elements. To do this we need to know the (row,column) pair ("Vertex Span Index") for each triangle vertex. This function returns a 3-tuple of vertex span indices for a given quad triangle. The functionEncodeRowColumnIndex()is used to map the (row,column) to a single integer, and callers can useDecodeRowColumnIndex()to recover the (row,column)