document possibility of 'eps-bytes+bounds for file/convertible
support eps-bytes+bounds for picts
add the --dvipdf flag to scribble

This adds a new back-end pipeline for generating pdf to
scribble, with the hope that included picts (e.g., those
generated by Redex) will look better when viewed with
on-screen pdf viewers

Current problem: the descent adjustment is messed up,
as can be seen by this example program's output:

 #lang scribble/base
@(require redex/pict
          pict
          redex/reduction-semantics)
@title{}
@(define-language L
   (e ::= (e e) x (λ (x) e)))
@(render-language L)

ff@(text "f" '(italic . roman) 20)ff

x@(text "x")y@(text "y")