cubetruss_corner() parameter clipthick could optionally be a vector correspondent to extents

[ToddRAllen]

This change would give control over placing a clip and how thick or no clip at all in each direction. Two cubetruss_corner objects can directly clip together without the use of separate uclips if one and not both have integrated clips and this change I think would allow that in a straightforward fashion while retaining compatibility for code now using this library module.

I've looked at the library code and believe I could implement this change properly. I am a retired professional programmer (video games) and have read the CONTRIBUTING.md guide but I have never worked on a github based project and may need a bit of guidance to successfully contribute.