Add proper UV coordinates to objects#11

All objects need to have sensible UV coordinates for texture mapping. We need to decide on a projection for spheres. UV coordinates for triangles may be derived from the barycentric coordinates implemented in PR #6. planes need to be able to describe a rectangular region to which textures are mapped, then have it repeated over the surface. Blob and mesh UV should derive from each triangle's mapping. Optionally, the UV coordinates of vertices could be added and used to map multiple objects onto one texture.