Fixes #20890
Add test for #20676

There was problem in this lines:

std::vector<cv::Point2f> resultingTriangle, polygon;
convexHull(points, polygon, true, true);

convexHull use same type for points and hull(variable polygon in this case). If the types are different, a problem occurs.
Using cv::Mat fix this problem.

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