TODO: Decide whether the following decision is sensible! Maybe we should
require all shapes be affinely transformable? Would be okay for polygons,
ellipses, and beziers. Maybe we should use homogeneous coordinates after all?
Also TODO, think about passing in triangulator to polygon builder and have
shapes triangulate internally. This would help in cases where triangulation
is necessary for calculation of other properties, so as not to have to
recalculate it...
In general, classes and interfaces which extend Shape should deal with
non-homogeneous vectors, in the interests of a simple and consistent
API. Shapes don't generally support affine transformations or related
operations, so there is little value in trying to make homogeneous coordinate
space a consistent part of the API at this level. Classes related to
Mesh, on the other hand, do deal with homogeneous coordinate space.