Fix compilation for OpenCV 4

[adujardin]

Using OpenCV 4 and Ubuntu (18.04) on default settings, the compilation fail with :

examples/tests/resizeTest.cpp:82:64: error: ‘CV_LOAD_IMAGE_GRAYSCALE’ was not declared in this scope
             op::Matrix opImg = op::loadImage(FLAGS_image_path, CV_LOAD_IMAGE_GRAYSCALE);

This simple fix replace the legacy OpenCV C API by the C++ one.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[peasant98]

I also had the same issue

[gineshidalgo99]

Thanks! But unfortunately, I can't accept it because it would break 2.X compatibility (lots of people still using that OpenCV as well). Instead, I've just pushed an equivalent fix that doesn't break compatibility with the old 2.X version. Thanks for the catch!!