Fix PDF output of clipped graphics with recent graphics package
The previous redirection of the senseless option "bb" to "viewport" in
PDFLaTeX output has been removed in a recent graphics package update.
This breaks documents, since clipped graphics silently stop displaying.
This change restores the previous output by using "viewport" instaed of
"bb" for non-PS/DVI output, while leaving PS/DVI untouched (where "bb"
and "viewport" behave differently.