Fix vertical offset of fonts like MinionPro

Fix vertical offset of fonts like MinionPro

For some reason, adding a LTR/RTL Override unicode character before a
MinionPro string increases the ascent of the line.

This creates vertical offset issues when painting if the ascent used
as reference is the one of the font. Use the QTextLine ascent instead
for better results.

This is a followup to 1bed76e2a.

Fixes (reopened) ticket #11284.