Fix border size in DT_IOP_BORDERS_ASPECT_CONSTANT_VALUE mode.

In this case we compute the size of the border using the larger border.
This is to make sure that the border will have the same size if the
picture is in landscape or portrait orientation.

We probably want this in 1.2. It is a really nice usability fix that I have been tracking since
some time. It is related to my watermark work, indeed I have seen that I had a larger space
between the watermark and border in landscape mode. I found the culprit.

Ok to merge?