Change latex render units from px (pdflatex specific) to bp (tex).

The px unit is a pdflatex specific adjustable unit that is 1 bp (big
point = 1/72in) by default.  This commit changes the latex renderer to
use bp which is a standard TeX unit equivalent to the default px
value.  This change allows .tex files generated by scribble to work
with other latex engines such as xelatex.

http://nwalsh.com/tex/texhelp/Plain.html#dimensions
http://tex.stackexchange.com/questions/41370/what-are-the-possible-dimensions-sizes-units-latex-understands

Here is a small test of using scribble and xelatex:

$ cat try.scrbl
@(require scriblib/figure redex/reduction-semantics redex/pict)
@(define-language L)
@(render-term L (term 1))

$ scribble --latex try.scrbl ; xelatex try

original commit: 0dfcf634ed29a9d6e9d99ea7bcd02121abd24a7b

collects/scribble/latex-render.rkt

@@ -324,7 +324,7 @@
                                                                (- (ceiling height) height)))]
                                               [fn (install-file (format "pict~a" suffix) bstr)])
                                          (if descent
-                                             (printf "\\raisebox{-~apx}{\\makebox[~apx][l]{\\includegraphics{~a}}}" 
+                                             (printf "\\raisebox{-~abp}{\\makebox[~abp][l]{\\includegraphics{~a}}}" 
                                                      descent
                                                      width 
                                                      fn)