The setting adds the "Custom Shape Wire Width"
option to the "Viewport Display/Custom Shape" section of a pose bone.
As the setting says, this controls how thick the wire is drawn in the viewport.
This is done by adding a geometry shader that makes two triangles out of a line.
The Anti-Aliasing is controlled by the setting
Viewport->Quality->Smooth Wires->Overlay in the user preferences.
## Artifacts
When increasing the line width, the lines start to separate at their vertices.
This comes from extruding each edge along the normal of its direction.
This could be solved by adding round caps in a later PR.
Pull Request: https://projects.blender.org/blender/blender/pulls/120176
f9ea64b0ba