OpenCV: upgrade, fix MinGW

Hello @vonreth and @cgilles :)

In Kdenlive, one point advertised in our latest release was the tracker filter, based on recent OpenCV-contrib code, that we ship with our AppImage. Our Windows users were quite upset not to have access to it.

I spent quite some time to get it to build in Craft with MinGW: it required several options tuning. I also disabled all internal 3rd party lib builds.

With "contrib" extra modules, OpenCV becomes a huge beast taking ages to build, eating much space, downloading things etc. So I chose to pick (whitelist) the very minimal modules we use: in Kdenlive we just need "tracking"; quick search in digikam made me think it uses "face". They pull several "core" dependencies (calib3d, features2d, flann, imgproc, objdetect, photo, plot, video). If I missed any, please tell so that I add it to the list.

Also, I hope the approach I used to download the "contrib" code is appropriate, I picked it from llvm-meta, but didn't setn "opencv-meta" with a version.ini... is it OK like this?

I hope this will improve the situation for everybody.

Cheers