[coretext] Unbreak glyph positioning in presence of notdef runs As discovered on Chrome Mac: https://code.google.com/p/chromium/issues/detail?id=452326 This was originally broken in: commit 5a0eed3b50629be4826e4e9428f2c3255195395d Author: Behdad Esfahbod <behdad@behdad.org> Date: Mon Aug 11 23:47:16 2014 -0400 [coretext] Implement vertical shaping