Fix vertical alignment of non-vertical glyphs

[JimBobSquarePants]

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Description

When I removed the offsetting of vertical glyphs during #432 I forgot about non-vertical glyphs. This PR reintroduces the behavior in a more correct manner.

Note. It's really hard to write unit tests for this stuff. I think we need to introduce some regression tests in the form of image comparison using ImageSharp.Drawing to render the result.

Rendered output

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src/SixLabors.Fonts/TextLayout.cs:541

• [nitpick] The variable name alignX is ambiguous. It should be renamed to horizontalAlignmentOffset.

float alignX = 0;