Generalize Closed Paths

[pasbi]

Having one boolean flag that determines whether a path is closed is not expressive enough.
The concept used to be inadequate (multi segment paths were forced to be all-closed or all-open).
But since we can join points and make much more sophisticated paths, the concept is completely insufficient.

I think we can drop the is-closed-property completely.
With the new joining-points feature, we can easily create closed paths without that.
However, the brush feature will stop working then because the path does not appear to be closed for the painter anymore.

Plus, it would be nice if the user could select the areas of a path they want to be filled.
Figma supports that (I haven't used it myself, but from the screencast it seems very easy to use and powerful).
I could imagine that some heuristics and clever programming are required in edge cases, when joints are modified.