affine_transform: increased shape of required input array slices

[m-albert]

This PR fixes a problem revealed by the tests of @martinschorb when using affine_transform within rotate in this PR #213.

Some choices of transformation parameters, which hadn't been covered in the tests so far, require a larger input array slice to compute the output chunks.

Only code change in this PR: +2 instead of +1:

        rel_image_slice = tuple([slice(int(rel_image_i[dim]),
                                       int(rel_image_f[dim]) + 2)
                                 for dim in range(n)])

I modified the tests to reflect the requirement for these larger input slices.