As part of adding OpenType support in Pango Thai module, I find some wierdness
of GPOS glyph positioning as described in Bug #141541, Comment #4, issue 2.
After trying moving the anchors around, I find the effect of the movement as
rendered by Pango becomes in opposite direction, e.g. when I move base anchor
up, the mark gets shifted down.

This makes me think there must be something wrong in Pango/FreeType, because the
font data is all OK when dumped with Fonttools/ttx.

One smell I gets is that the OTL_PositionRec uses coordinate system in which Y
increases in up direction, while PangoGlyphGeometry may be using screen
coordinate, in which Y grows down the screen.