This also cleans up a doc comment.

    This is specifically for line-line intersection, where there is some
    special logic in fonttools that we were not replicating. Specifically
    this deals with cases where one segment ends either directly on another
    line, or on one of the end points.

    This brings over a few small & specific bits of logic from fonttools to
    try and match some particular edge cases; I verified that this makes
    some things on crater improve, and makes nothing worse.

Better handling of line-line intersection with low `t` (in open corners)