Added rounded rectangle support for the GDI-Geometry-Provider

Adds rounded rectangle support for the GDI-Geometry-Provider.
I needed it and didnt want to use SharpDX :)

![roundedrectangle](https://f.cloud.github.com/assets/3882447/521252/b34dd49a-bf8c-11e2-8d90-ddbb56a231b7.png)

P.S. This is my first pull request, correct me if i am doing wrong.