Fix PDF output of clipped graphics with recent graphics package

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.

Fixes: #7910