Returns the four corners of a rectangle

`corners(window)`

A list with two components `x`

and `y`

, which are numeric
vectors of length 4 giving the coordinates of the four corner points
of the (bounding rectangle of the) window.

This trivial function is occasionally convenient.
If `window`

is of type `"rectangle"`

this returns the
four corners of the window itself;
otherwise, it returns the corners of the bounding rectangle
of the window.

# NOT RUN { w <- unit.square() corners(w) # returns list(x=c(0,1,0,1),y=c(0,0,1,1)) # }