Do not mark image edges in 'subpixel' mode of find_boundaries

[grlee77]

Description

This PR closes #5432, following the suggestion by @firerain42. If there is some reason we want to keep the option with the old behavior of treating the boundary of the image as an edge, let me know. At the moment I am considering this a bug fix and so have not set up a deprecation cycle.

It seems none of the other mode options to find_boundary mark the edge as a boundary, it was only the subpixel mode. A test was added that demonstrates this for all modes.

@firerain42, if you are interested in being credited as a co-author on the commit message, please provide a name and email address and I will add a comment for that

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[rfezzani]

Thank you @grlee77, I'll wait for @firerain42 answer if he want to be sited as co-author before merging 😉

[firerain42]

Thanks @grlee77 for implementing the fix and @jni and @rfezzani for the review.

If it's no hassle for you, I would like to be a co-author (Lukas Petrich firerain42@googlemail.com). Thanks.

[grlee77]

Thanks @firerain42, I added the text for co-authorship while creating the merge commit. You can see it here:
44ff414