This is causing vertical glyph positioning to move in the wrong
direction e.g. marks go below base glyph instead of above. Seems to be a
regression introduced in:

commit 83dac189
Author: Tavmjong Bah <tavmjong@free.fr>
Date:   Tue Nov 3 13:19:36 2015 +0100

    Rearrange code to make handling of baseline clearer.

    (bzr r14430.1.3)