Make the Pen tool show a path being closed by drawing a filled overlay when hovering the endpoint

[Keavon]

Add a visual indicator when the Pen tool would close a path by showing a filled interior to the path (using the same translucent blue as we use for rectangular/lasso selection) when hovering over the endpoint that will make the shape become closed. This needs to work with any subpath out of potentially multiple. It doesn't yet have to work with mesh vector (enabled in the app preferences), but that would be a bonus.

[c-mateo]

#2521 provides the required functionality but doesn't work properly with vector meshes