diag method
... for the specified diagonal (d=0 - the main diagonal,
0 - a diagonal from the upper half, <0 - a diagonal from the lower half)
Implementation
UMat diag({int d = 0}) {
final p = calloc<cvg.UMat>();
cvRun(() => ccore.cv_UMat_diag(ref, d, p, ffi.nullptr));
return UMat._(p);
}