I'm little confused about parameter font variations of [add_font_instance](https://github.com/servo/webrender/blob/75aa8444754675870e3974b83574eb9da7bcf812/webrender_api/src/font.rs#L147), and in almost all examples, we just pass an empty vector to it. Recently, I found [create CTFont will lose font variation](https://github.com/servo/core-foundation-rs/pull/430), and as @jrmuizel said, we create native font with postscript name and create data font with font descriptor[(and font variation)](https://github.com/servo/webrender/blob/062b848565ea355c3597c27348be05468c420636/webrender/src/platform/macos/font.rs#L435). I thought we can get font variations only if we got `CTFont/CGFont`, right?