Support (Convex) Minkowski Sum construction

[rmanoka]

Minkowski sum is a sort of generalization of buffering. The buffering of a geometry G upto radius r is same as minkowski sum G + B, where B is a circular area centered at the origin, and has radius r. This sum can be calculated via boolean ops when B is a convex polygon.

This should address #641 to some extent: taking minkowski sum of G with appropriate approximation of a circular region is decent approximation to buffering. I don't know if the output is aesthetically nice (because of the approximation, there may be some sharp edges), but it satisfies the functional requirement of buffering: any point within distance r from the original geometry is contained in the output.