Computer Vision based Alpha Matting (#2306)

* Computer Vision based Alpha Matting Code

alpha matting code

This is a combination of 3 commits.

removed whitespaces

addressed issues raised in the PR

removed whitespaces

* removed global variable

* incorporated changes suggested by second round of review

* updated build instructions

* changed to OutputArray

* removed whitespaces

* alphamat: fix bugs triggered by assertions of Debug builds

* alphamat: fix documentation

* alphamat: coding style fixes

- get rid of std::cout
- remove clock_t
- drop unnecessary cast: float pix = tmap.at<uchar>(i, j);
- global 'dim' => 'ALPHAMAT_DIM'
- fix sample command line handling

* alphamat: apply clang-format

* clang-format fixups