Diacritics should not overlap

Submitted by Denis Jacquerye @moyogo

Link to original bug (#322234)

Description

If a font does not have OpenType GPOS and GSUB table defining where the diacritics should go, they just end up in the same position everytime. Independently of which characters they are composed with.

Instead they should either stack upwards or downwards, for above diacritics and below diacritics. Qt does something like this. Pango should be able to heuristically place diacritics.

Version: 1.10.x