Feature request: add vertex to existing pixel mask

[emilyrbowe]

When editing existing masks, a user is able to edit an existing vertex, but there's no way to add a new vertex. When a user has a mask selected and is hovering over the mask line, a dot pops up that makes it seem like they will be able to add a point to the existing mask, but it actually starts a new pixel mask (see below).

This functionality is very important when trying to mask pages of an atlas that will be eventually mosaic-ed together.