added local transform to bezier contexts

- makes it easier to add primitives in a `with bezier` block since you
can apply transforms to them *as they’re added*
poly can now draw regularized ‘star’ polygons

- poly(x, y, r) draws a square with sides of 2*r
- poly(x, y, r, sides=n) draws a regular n-gon
- poly(x, y, r, points=n) draws an n-pointed star
add a test for star-polygons
added an update() method to Grob

- you can now batch up a sequence of `grob.prop = val` lines into a
single call: grob.update(prop=val, prop2=val2, …)
the bezier-local transform should be identity

- not a copy of the existing transform, duh