[metapost] different bounding boxes for identical figures

[Sanjoy Mahajan]

Just solved the problem.  It was subtle.  

I had noticed the bug with the bbox when using mpost 0.641.  I just
upgraded to 0.901 and reran mpost on d.mp.  It failed only for d.mp,
not for any copies of it.  Eventually I had the idea to look at d.fls
(which is why I put -recorder in the mpost invocation) and see what it
was reading.  The INPUT list included 'd.mpx', which had this first
line:

% Written by DVItoMP, Version 0.64/color (Web2C 7.5.4)

When I deleted d.mpx and reran 'mpost d.mp', it produced the correct
figures, and d.mpx now has this first line

% Written by DVItoMP, Version 0.9/color (Web2C 7.5.3)

I wonder whether mp should refuse to read a .mpx file made by a
previous version?

-Sanjoy

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