Fix \pbo

Fix \pbo

b61d260b attempted to add support for \p scaling to \pbo.
However, while it fixed the exact sample originally reported,
it broke \pbo with other \p values, including \p1, by confusing
the direction of scaling and the units in which \pbo is measured.

In addition, the descenders assigned to drawings have always
had a wrong sign, causing lines that contain drawings with \pbo
to be shifted in the wrong direction: negative \pbo raised bottom-
aligned lines, while positive \pbo lowered top-aligned lines.

All errors and fixes have been confirmed with VSFilter.