OpenCV C-API support

[Dima Kogan <dkogan@debian.org>]

Hi all.

So it looks like the OpenCV people have pretty much abandoned their C
API, to the point where trivial applications no longer build:

  https://github.com/opencv/opencv/issues/6221
  https://github.com/opencv/opencv/issues/8658
  https://github.com/opencv/opencv/issues/10246

They're uninterested in keeping this running, but this breaks a lot of
legacy applications. I think we (Debian) should keep this working if
possible.

There're two approaches:

1. Update the packaging to (at least) make OpenCV2 available in sid, and
   at best also make it co-installable with OpenCV3.

2. Patch our build of OpenCV3 that works with the older API. Right now,
   this actually is simple (I think). We'd just need to move the
   definition of cvRound() to C header instead of a C++ one. I'm sure
   there're details to be worked out, but it LOOKS easy. With time,
   these updates could become difficult, but not yet.

I favor option 2. Any strong opinions either way? If I don't hear any
complaining, I'll get the patches ready, and then talk about an actual
upload.

dima