backend change needed: render patch without boundary

I think this would be a good idea. Having lines of width

    > 0 be invisible would satisfy the principle of least
    > surprise. (If people really would need the current
    > PostScript backend behaviour, maybe this could be exposed
    > through a different API?)

Despite my earlier concerns, after thinking about it a bit, my
preference at this point is to not support the postscript behavior. I
think the front-end API should strive for consistency across backends,
rather than supporting backend dependent behavior. So let's go with
linewidth=0 is invisible, unless someone feels strongly otherwise.