This commit makes CoreText behave a lot like FreeType where we set the
variation axes on the deferred face load. This fixes a bug where the
`slnt` variation axis could not be set with CoreText with the Monaspace
Argon Variable font.
This was a bug found in Discord. Specifically, with the Monaspace Argon
Variable font, the `slnt` variation axis could not be set with CoreText.
I'm not sure _exactly_ what causes this but I suspect it has to do with
the `slnt` axis being a negative value. I'm not sure if this is a bug
with CoreText or not.
What was happening was that with CoreText, we set the variation axes
during discovery and expect them to be preserved in the resulting
discovered faces. That seems to be true with the `wght` axis but not the
`slnt` axis for whatever reason.
e08eeb2b2a