I propose adding noexcept to default constructors of: Mat, UMat, Mat_, MatSize, MatStep
I discovered they were missing it, yet could have it, while I was experimenting with solutions for another issue.
This continues my work I started in the ocl namespace #19571

As this is a new feature, not a bug fix, and requires C++11, this is for the 4.x branch of OpenCV.
Locally tested with all unit tests passing.

System information (version)

• OpenCV => 4.5.1 eb82ba3
• Operating System / Platform => All
• Compiler => All

Detailed description

default constructors of: Mat, UMat, Mat_, MatSize, MatStep
all do not have noexcept. Yet, they are capable of such. Having noexcept (or the macro CV_NOEXCEPT) enables wider and easier use in STL, in derived classes, etc.

Steps to reproduce

Code review. See default constructors for: Mat, UMat, Mat_, MatSize, MatStep

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  forum.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there