Fix arrows requiring a radius to be visible

[Wumpf]

Fixes #1708

.. that is the immediately user-facing problem this solves. But in trying to solve this properly I had to make the line renderer a bit more predictable: Previously, line caps would extend the line. Now they stay within the bounds of the previously determined line positions!

2D Demo before - note how caps make the lines longer:

and after - all the same length now:

rounded rectangles are still rounded (requires outward extending the rounded cap):

What an outline around it looks like (clock example):

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
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[Wumpf]

oops. not actually the same length yet on all combination. Thanks for pointing that out @nikolausWest 😅

[emilk]

For arrow tips I think it makes sense that the end-point is at the tip.

For rounded ends I find it more intuitive that the end-point is in the center of the hemisphere. That way one can easily combine multiple capsules together into a smooth line-strip with concentric hemispheres.

[Wumpf]

I actually broke rectangles I noticed a bit too late. I have now an option to configure the outward extensibility which addresses both that issue and your comment :)
(at least partially, as I don't allow mixing that yet, could easily extend it though)
View doesn't use rounded caps anywhere else yet, so punting on that question

[Wumpf]

there we go! overlapping could still be handled better, but let's not go there now

[emilk]

Looks good, but I think the code could be simplified quite a bit!

[Wumpf]

thanks for the input! Most of the stuff suggested doesn't work as-is because of some detail (typically on how cap triangles are flagged) but that does stress the point that this got a bit too hard to follow again. I'll try to make it simpler!