Fix rendering font size and overline position.

[JimBobSquarePants]

Prerequisites

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• [ ] I have provided test coverage for my change (where applicable)

Description

Fixes a regression where the current run font size was not being used for rendering. Also fixed overline position to match CSS

[JimBobSquarePants]

I've already directly tested this locally against https://github.com/SixLabors/ImageSharp.Drawing/pull/211