[metapost] different bounding boxes for identical figures

[Sanjoy Mahajan]

> If you are up to it, the source distribution of metapost is here:
>   https://foundry.supelec.fr/frs/?group_id=13

I'll do that.  I had compiled it once before, to check whether the
"graph: no spline in log-log plot" problem I reported (15 Jan 06) went
away.  It didn't, which was not a surprise in retrospect since
graph.mp didn't change between 0.641 and 0.901.  This time I'll
install it over the 0.641 version for keeps.

-Sanjoy

`Never underestimate the evil of which men of power are capable.'
         --Bertrand Russell, _War Crimes in Vietnam_, chapter 1.