This is only used in servoshell, even though it was plumbed through
script previously. It's just about how the `RenderingContext` is set up,
which is something managed entirely outside of servo itself.
In addition, make the name of `servo_shell_preferences` in `app.rs` more
consistent with the rest of the codebase.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>