PDL::OpenCV

This is an attempt to interface OpenCV from perl and PDL.
There is a C abstraction layer
because of conflicts between perl and OpenCV (C++) headers.

The idiom used is that OpenCV Mat objects are created only to be used
to wrap PDL ndarrays' data, which simplifies object lifecycle management.

Currently this is compiled for OpenCV 4.2 and 4.5. At least between
major versions, the API is changing.

OpenCV has a multitude of functions and classes. Wrappers are generated
from the data that Python bindings are generated from.
