Add font embolden

[jrmoulton]

This is basically just a direct port of the swash implementation of font embolden.

It does seem to have about twice the "strength" of the swash implementation though and I haven't been able to figure out why.

[jrmoulton]

Need to investigate:

• Strength double
• Doing embolden in stroke expansion on the GPU

[jrmoulton]

This is now working.

It has path expansion/stem darkening working on the GPU and properly works with Bevel, Miter, and Round joins. These are all configurable.

The changes are implemented in both the cpu and gpu flatten shader.

[raphlinus]

A quick update, this was discussed in Renderer office hours, also see transcript of the discussion.

This would be great to have in some form, but the big question is review bandwidth as it's a fairly major change and has a risk of breaking things. We haven't forgotten about it, but need to decide what to do.

[jrmoulton]

I have updated this PR to allow the embolden to take a vec2 for separate emboldening of x and y.

I will add some comments to the PR to highlight parts that need particular review and open questions.