fix compatibility with modern opencv

[sandsmark]

[robwhess]

Thanks for the PR @sandsmark. As you may have noticed (especially given the nature of this PR), I haven't really been maintaining this repo, and I'm sorry to say that I don't really have the bandwidth to evaluate a PR like this. If anyone from the community wants to give input on the PR here, please feel free, and I will take it under advisement.

Note that the SIFT functionality included in OpenCV is already based on my code, and that should hopefully be straightforward enough to use with the most recent versions of OpenCV. I'd encourage you to give that a try.