Add primitive support for roto:

- Bezier interaction only for now (the rectangle/ellipse don't work
yet, but it's easy to implement them based on bezier)
- The mask rendering doesn't support anti-aliasing nor feathering yet.
- The viewer gui doesn't have any icons yet, just text on buttons
- The roto node settings panel isn't implemented yet
- Undo/Redo isn't implemented yet
- Serialization of the splines isn't implemented yet.
- A bug in the smooth functionality when the tangents are equal to a
control point.