fix: explicit gpu deallocation

[Krasner]

What was changed & why

Fixes: #

Changes

Testing & Verification

Additional Resources

[Krasner]

(Same comment as in Ryan-Millard#449)

@Ryan-Millard there is still some weird gpu deallocation issue with node js.
I think part of it is that the C++ code never explicitly deallocates the GPU - instead it relies on RAII to automatically clean up the GPU instance - which never happens on the JS side because of EMSCRIPTEN_KEEPALIVE

I put a PR review on my own branch: #5
Can you take a look and check if it makes sense. I've added a img2num::terminate() call that explicitly calls the GPU class destructor.

With this I think I reliably don't get any more gpu deallocation errors.