Optimise GPOS mark generation

When Glyphsapp generates a font, it makes a single mark lookup where all base glyphs and mark glyphs are positioned against each other. When mark glyphs are added to base glyphs which don’t have the necessary mark anchor, they are positioned at 0,0.

I understand this solution may have been used to minimise complexity and allow the use of 1 lookup for all mark positions.