Feature Request: Get envelope of path stroked with arbitrary pen

It would be of immense use if it were possible to get the path of the
outline of some arbitrary pen stroke.

Since metapost already outputs bezier curves, instead of bitmaps, it would
seem that the machinery to compute the envelope is already in place, so it
shouldn't be too difficult to expose this as an operator in the language
itself.

-- 
Thanks,
Kevin

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http://tug.org/metapost/