OpenCV gapi module

[cocoa-xu]

Based on the contents of pyopencv_gapi.hpp, it seems that the binding code will need to call an Elixir function and fetch the result.

This perhaps could be done if we send related ERL_NIF_TERMs to a separate Erlang thread and send the result back. But it's not easy to properly design and write the whole thing.

Maybe it's not worth the effort... I probably won't port this module anytime soon.

This module is highly likely to be excluded from evision 0.1.0.