Disable ligatures in Pango. Using the CoreText backend we can't visually move the caret
position inside ligatures, so the caret position gets offset when you try to key around
inside a GtkEntry that contains any ligatures, e.g. "fluid" has "fl" ligature so the
caret is offset by 1.

https://bugzilla.xamarin.com/show_bug.cgi?id=15787