Draw joints for closed shapes without redrawing the first line segment

#22085 added `Gizmos::lineloop_2d` for drawing closed shapes, and made all the closed shapes with sharp corners use it. This fixed #22081, in which the joint was missing on the first corner of closed shapes.

The simple solution in #22085 was to redraw the first line segment at the end of the line strip. This works, but involves unnecessary drawing. A better solution would be to draw the first joint without overdrawing the first segment.

Start by reading the implementation of Gizmos::lineloop_2d and the closed-shape changes from #22085, then inspect how the first joint is currently produced. The work is done when closed shapes retain the first-corner joint without redrawing the first line segment, while preserving the fix for #22081.

Written by the indexing model from the issue text.