Type 3 glyph boxes used the font matrix's scale only (its first element,
with the FontBBox's height), leaving out translation and skew. The
renderer applies the whole matrix, so a glyph the font matrix moves lay
outside the box redaction and the final verifier tested: a mark over the
drawn glyph neither removed it nor saw it left behind.
The glyph box now also covers the FontBBox through the whole FontMatrix,
for Tf and for an ExtGState font; the union with the old box keeps
every position the old test covered.