Draw bounding boxes of objects and draw Voronoi below them

[romsson]

The user can draw rectangle overlays on objects and then the Voronoi is calculated based on the boundaries of this object (Related to #67

Each rectangle can then be used as a mask to re-draw the selected object above the Voronoi area. This may not be rendered perfectly as all objects are not rectangle, but may be a first approach to be improved later.