The previous code was calculating the rounded up version of the
difference between the unhinted ascender and uninted descender. This
however would only result in the correct height if both unhinted
extentss together only differ by less then an a Pango unit from their
hinted counterparts. Otherwise the resulting height would be 1 unit too
short.

Fix this by using the difference between the hinted extents as height.

Fixes: #626