Merge pull request #28907 from chacha21:more_autobuffer

More use of AutoBuffer #28907

When possible, AutoBuffer should be faster than std::vector<>, and should not be worse if it requires a heap allocation rather than a stack allocation.

### Pull Request Readiness Checklist

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

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [X] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [X] 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

modules/video/src/optflowgf.cpp

@@ -822,7 +822,7 @@ private:
     float  m_ig[4];
     void setPolynomialExpansionConsts(int n, double sigma)
     {
-        std::vector<float> buf(n*6 + 3);
+        AutoBuffer<float> buf(n*6 + 3);
         float* g = &buf[0] + n;
         float* xg = g + n*2 + 1;
         float* xxg = xg + n*2 + 1;