Fix arc border with circumference over 2*PI

[kurkle]

Fixes: #6171

Demo

[nagix]

Looks good in case 0 <= circumference <= 2*PI and 2*PI < circumference <= 4*PI, but appearance is not consistent when circumference > 4*PI. How about looping this part (endAngle - startAngle) / (Math.PI * 2) times?

[kurkle]

Loop implemented, did some more refactoring to keep CC (and me) happy 😄
Demo updated.

[etimberg]

Instead of a loop would doing % 2PI be more efficient?

[kurkle]

@etimberg that was my initial approach, but with transparency it is not consistent with circumference > 4*PI.
Not sure if anyone is going to use circumference that high anyway, but now its drawn consistently.

[nagix]

When borderAlign is set to 'inner', the result doesn't look correct because the border appears at next to the end angle instead of at the start angle.

Similarly, when borderAlign is 'center', the border should also be drawn and visible at the the start angle as the background is transparent.

I think we need to fill and stroke arcs multiple times if the circumference is greater than 2*PI. The first arc would have an open path (open at the end edge), the following arcs would have 2 open paths for outer and inner circumferences and the last arc would have an open path again (open at the start edge).

[kurkle]

Fixed drawing of borders. But in 2 steps instead of 3, first fully open full circles and then fully closed partial.
Demo updated.

[nagix]

Thanks for the fix. When borderAlign is 'inner' and the overlap is very small, the result is a bit odd, but I know there is no good way to fix. The other parts look perfect, so I think it's ok now.