Should IOSurface be a part of core-graphics-types?

[kvark]

Similar to #479, but for IOSurface.
Triggered by gfx-rs/metal-rs#223
io-surface crate has a whole lot of dependencies, and we just need an IOSurfaceRef.

[jrmuizel]

How about something like #484 instead? That reduces io-surface's dependencies to the same as core-graphics-types

[kvark]

You are suggesting this for io-surface crate? I don't think this would be a good solution, because it would still be breaking often (because each optional dependency can require breaking), so version on io-surface are still going to climb up faster than on metal-rs. I want metal-rs dependencies to be mostly set in stone in terms of compatibility.