Add test 'border_overlap' theming to somewhat control overlapping of shapes and their borders

[MyreMylar]

Artefacts reported in #659 are hard to address universally - we want to have soft corners, but also to support transparency and also support a wide variety of curves and have flexibility about where shapes will be.

A potential solution is to pass over control of the 'overlap' parameter to users so they can tweak it for their situations.

Previously it was hard coded to "0" for rounded corner buttons and to "1" for ellipse shaped buttons. Clearly I've tinkered with various values in the past to try and solve the various artefacts, but no value solves all problems.

In this PR 'border_overlap' is added (but not documented yet) as a miscellaneous theming parameter and I've set it to "1" by default everywhere as I think this is probably the most widely applicable default value.

I'll see how the default value of 1 goes down in wider testing in the next release and if the hidden theming value is helpful (or not) before documenting it in the future.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.77%. Comparing base (4caccad) to head (9db2a81).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #660   +/-   ##
=======================================
  Coverage   95.77%   95.77%           
=======================================
  Files          90       90           
  Lines       14095    14105   +10     
=======================================
+ Hits        13499    13509   +10     
  Misses        596      596           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.