Fix underline position on Liberation Mono

Ensure we don't round a underline position beyond the descent in our
eagerness to avoid underlines too close to baseline.

Task-number: QTCREATORBUG-15851
Change-Id: I9a29447bbcb938b7e9fb29d52fd392a1340d07c5
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>