Progress so far

Here's a screenshot of a newly created mask in the UV/Image Editor:

Roto2D in action

It shows the panel on the left which features a list of masks. The mask itself shows the different types of points that are available: a simple point, a point with one handle or a point with two handles. A point with two handles can have an angle between the handles or the handles can be symmetrical.

All the points and handles can be moved around individually or, using the shift key or a rectangle selection to select multiple points, as a group. With the shift key you can also change the angle between handles or pull a new handle from a simple point or a point with one handle.

So, the mask editor is pretty much functional. The next step is to generate the raster image mask.