Problem 1704. Triangular matrices in 3D array

Given a 3D numeric array x, return an array y of the same size in which all entries to the right of the main diagonal are zero for y(:,:,i).

Example

If

x(:,:,1) = 1 2 3
           4 5 6
           7 8 9

x(:,:,2) = 1 4 7
           2 5 8
           3 6 9

x(:,:,3) = 1 2 3
           1 2 3
           1 2 3

then

y(:,:,1) = 1 0 0
           4 5 0
           7 8 9

y(:,:,2) = 1 0 0
           2 5 0
           3 6 9

y(:,:,3) = 1 0 0
           1 2 0
           1 2 3