diffenator: use blackrenderer

[m4rc1e]

@simoncozens I see we've got our own cool draw_text function, https://github.com/googlefonts/gftools/blob/diff2-fuzz/Lib/gftools/diffenator/ft_hb_shape.py#L9-L18. Perhaps it's better if we use blackrenderer so it'll allow us to also gen colr v1 bitmaps?

I've just quickly modified our function so it uses blackrender and there's a few differences we need to consider:

left: blackrenderer, right: ft_hb_shape.draw_text

• for non-color fonts, colors are inverted
• draw_text includes the left side bearing but not the right (did you mean intentionally want this?). Blackrenderer includes both sidebearings
• blackrenders output array is for RGBA pixels. Ours is just for greyscale

If none of the above are issues to you, I shall proceed and integrate blackrenderer.

[simoncozens]

Looks good. We will probably have to fix the diff function but I think go for it.

[m4rc1e]

Cool, I'll proceed. Since the diff function uses numpy arrays, it works from the get go

>>> a = np.array([
        [[255, 255, 0],[255, 0, 0]],
    ])
>>> b = np.array([
        [[255, 255, 0], [255, 0, 0]]
    ])
>>> print(img_diff(a,b))
0.0

>>> a = np.array([
        [[255, 255, 0],[255, 0, 0]],
    ])
>>> b = np.array([
        [[255, 255, 0], [125, 0, 0]]
    ])
>>> print(img_diff(a,b))
0.28

[simoncozens]

This issue is obsolete with the new diffenator2.