Fix automatic overhang threshold

[hyperair]

We should be supporting perimeters that are overhung further than half a
perimeter out, rather than two times the perimeter width.

Fixes: #2068

[curiouspl2]

+1

[lordofhyphens]

@hyperair How does this work with the default spacing (which is one support width from the model) for support?

[hyperair]

I remember it working fine -- the only thing it changed was whether or not some things got supported. But I got tired of keeping track of the diff, so I stopped using it altogether.

[bubnikv]

I believe this fix is correct, though I may be wrong and I don't understand the reasoning of @alexrj.

[lordofhyphens]

@hyperair could you make the width overhang (relative to perimeter width) configurable instead with a default of 0.5?

[lordofhyphens]

I extended it in #3752. Closing this PR.