allow to select skia as blackrenderer backend

[anthrotype]

Blackrenderer, the library used by FontGoggles to render COLRv1, allows to optionally use Skia as a backend in alternative to CoreGraphics on macOS.
Sometimes it could be useful to enable that while previewing a font inside FontGoggles, e.g. to test potentially different renderings across backends.
Since skia-python already has wheels for Mac, I believe it should be straightforward to allow that.

[justvanrossum]

I would not be opposed to this. However:

• I don't know how to draw with skia into a CoreGraphics context. Presumably it needs to go via a pixmap? That may not be 100% straightforward, as the FontGoggles drawing code is resolution agnostic.
• We need to be careful that skia-python is imported lazily, as FG has much wider macOS support than skia-python has.

[anthrotype]

I see, thanks. I'll take back the "straightforward" then :)