Eliminate some warnings

Depends on https://github.com/mozilla-servo/rust-fontconfig/pull/19, https://github.com/mozilla-servo/rust-harfbuzz/pull/21, https://github.com/mozilla-servo/rust-hubbub/pull/23, https://github.com/mozilla-servo/rust-layers/pull/68, https://github.com/mozilla-servo/sharegl/pull/29, https://github.com/mozilla-servo/rust-mozjs/pull/85.