Fix "wobbly" rotation of some polygons

[hsharrison]

Rotation works based on the center of the bounding box, which changes with rotation. As a result, triangles wobble as the rotate, as evident in graphics_demo.py. This is probably worse with non-regular polygons that are oddly shaped.

One solution is to add a keyword argument to Shape.rotate, perhaps use_true_center, that will override the bounding box behavior.

The issue is more complicated for rotation due to the angular_velocity attribute. Perhaps a boolean flag attribute use_true_center_when_rotating?