[ot] Hide default-ignorables before finishing off positioning For example, fixes the following sequence with Arial XP: 628 25cc 651 25cc 64e 3a 20 628 651 34f 64e 628 25cc 64e 25cc 651 3a 20 628 64e 34f 651 Discovered as part of: https://bugs.freedesktop.org/show_bug.cgi?id=85873