wb: disable ligatures and pairwise kerning while changing fonts
wb: interpret unknown escapes as normal characters
ren: italic correction escapes

This patch also allows troff font description files to include the
bounding box of the glyphs; the third column in the charset section,
which contains glyph widths, may include four more numbers just like
groff.  However, in neatroff these numbers specify glyph bounding box.