Polygon Get Vertices Issue

Hi,

I wasn't able to properly retrieve the vertices for a polygon shape due to some operations not being supported by the cpVect class (or Vec2d), so I hacked in a solution (check shape.pxi and core.pxi). Before you test my build please run the kivy_demo.py in the examples folder to see if your build will work, hopefully it should raise some errors and then test my new build.

Thank you for you time, I'm happy to answer any questions.