Boolean geometric operations

[kneitinger]

Currently nightgraphics' Shaped triat has a difference operation. In addition to that it should have union and intersection.

Tasks

• Add union operation to Shaped trait
• Add intersection operation to Shaped trait (this very well might share a lot of code with difference)
• Consider returning a Result as some operations do not make sense. For example, what does a the difference of 2 non-closed shapes imply? What about the union of a circle and a line?