Pango has a PANGO_SCALE value of 1024, making it really happy when fonts have a
cadratin value of 1024. Unfortunately, AHEM uses 1000, leading to rounding
errors.
We now use weasyprint.otf everywhere, with 1024. It breaks the exact 20/80
ratio between descent and ascent values, so that we need to use approximate
values for tests relying on an exact position of the baseline.
All the tests should pass, or be explicitely marked as failing.
