Ensure tessellation of polygon2 returns a collection of triangle2

[dgtized]

The return type of ITessellate tessellate for Circle, Triangle2, and Rect all
return a collection of Triangle2. However, calling tessellate on a polygon returned a
collection of point triplets. This patch ensures uniformity in the return type
for tessellating 2d shapes.

[dgtized]

Note that it appears that all of the 3d shapes return g/faces (which is a collection of Vec3 points), except for geom/quad which returns a set of Triangle3. So it might be useful to ensure uniformity there too.