Computes (x,y,z) components of quad centers (centroids).
The (x,y,z) coordinates of vertices are packed into the specified
array xyz. The number of vertices is nv = xyz.length/3.
Quads are specified by sets of four vertex indices (i,j,k,l), one
set per quad, packed into the specified array of integers ijkl.
The number of quads is nq = ijkl.length/4.
The returned array of packed (x,y,z) coordinates of quad centers
has length equal to ijkl.length. Each center (centroid) is the
average of the corresponding quad vertices.