Dash iterator zero-length LineTo noise

[RobertBrewitz]

If/When #550 is merged this would be a follow-up issue.

When a dash transition lands exaxtly on a path vertex, the iterator emits a zero-length LineTo at the corner instead of cleanly transitioning to the next dash.

Example:

let mut path = BezPath::new();
path.move_to((0., 0.));
path.move_to((3., 0.));
path.move_to((3., 3.));
let dashes = [3., 1.];
let result: Vec<PathEl> = dash_iter(path.into_iter(), 0., &dashes, true).collect();

// Current noisy output: MoveTo(0,0), **LineTo(3,0), LineTo(3,0)**, MoveTo(3,1), LineTo(3,3)
// Expected output: MoveTo(0,0), LineTo(3,0), MoveTo(3,1), LineTo(3,3)