further work on Ptcloud module

[berak]

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

trying to take over, where #2584 left it.

added sprt stopping, preemptive segmentation, python bindings and tests

[berak]

notes / todo / rfc

RTSP stopping is very useful here, and the USAC implementation from calib3d works nice.
however, i could not use the interfaces (needed to copy the code) because:

• they're neither public (header is in calib3d src) nor are they exported (win dll) [minor]
• they don't let me collect inlier indices (needed for local optimization later) [major]
• the rtsp verifier run its own per-point loop, and neither collects inliers, nor errors (for ransac or msac), also we cannot use a index mask for already segmented points here

i finally mimicked the Error interface from USAC, a distance function per point. sadly this is hard to low-level optimize (simd,parallel,..), like this is done in pcl.

some stats (timing, inlier counts) in the interface would be nice, too

[berak]

superseeded by opencv/opencv#21168
(which has much better USAC integration)