The old code would simply add advances and store as a float with
individual glyph positioning using floor() based on the advance +
offset. This breaks rendering of infinite length ligatures at some font
sizes as they either under or overflow. Rounding the advances seems to
fix it. Dont know if it will have other ill-effects for non infinite
length ligatures. If it does we can test total ligature length and
use rounding only for long ligatures.
Fixes#3896
55319cd6d6