Create an example of using with leptonica's c-api

[ccouzens]

It needed support to be able to get the mut pointer from leptonica-plumbing

https://github.com/ccouzens/leptonica-plumbing/commit/4a721e7eb107e0af634d4f3d1218a441f2ad729a

And for leptonica-plumbing pix to self reference

https://github.com/ccouzens/leptonica-plumbing/commit/545708cf8fc3da070852bd4a1f008324fd1a562b

It uses this so that methods can take AsRefLeptonicaPlumbing::Pix which both leptess and leptonica-plumbing satisfy.

Addresses https://github.com/houqp/leptess/issues/59