polygon center, circle rendering

Added a method to Polygon that calculates the barycenter, which I can
use for physics calculations. It probably has room for optimizing but I
can't be bothered right now.

Circles are rendered as triangle fans of arbitrary detail levels. I will
probably add methods to render circles with an automatically calculated
detail level based on the circle size but not now.

The temporary stand-in "camera" has been tweaked so I can see stuff that
is drawn in the scene using GL.vertex2() commands, so now I don't have
to use 3D coordinates at all except for the camera. And that's good
because I plan to use them for the camera.

Renamed Graphics to Artist because that's an awesome name, and took off
the 2d from Transform and ArtVector. Of course they're 2D. It's a 2D
engine.