Make text always align with font base line

Summary:
When font uses other fallback font, the base line may change to make all
text on the same baseline. But when we render the text, we should always
follow the baseline from FontMetrics to avoid text baseline jump up and
down depending on the text in the line.

Screenshot before and after.

Test Plan: Text manually with some mixed text.

Reviewers: rjvbb, dhaumann, cullmann

Reviewed By: cullmann

Subscribers: brauch, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D29789