Open a background non-focus-stealing window for tests on OS X.

Use false for the glfw::Visible window hint to open a background window
on OS X. This requires an upgrade to glfw 3.0.4 in order for this to
also not steal focus.

This requires adding a new parameter to WindowMethods<A>::new.

Fixes https://github.com/mozilla/servo/issues/2363. r? @larsbergstrom