Disable default link feature on core-graphics-types

When the `link` feature was forwarded to `core-graphics-types` in its
update in gfx-rs#297, it wasn't taken
into account that  the feature was enabled by default in
https://github.com/servo/core-foundation-rs/pull/608, leading to the
`link` feature to always remain enabled even if `default-features` are
**disabled** on the `metal` crate.

Disable the `link` feature by default on `core-graphics-types`, and rely
on our (default-enabled) `link` feature to turn it back on from inside
`metal`.

Cargo.toml

@@ -28,7 +28,7 @@ mps = []
 link = ["core-graphics-types/link"]
 
 [dependencies]
-core-graphics-types = "0.1.3"
+core-graphics-types = { version = "0.1.3", default-features = false }
 bitflags = "2"
 log = "0.4"
 block = "0.1.6"