Extended Latin and Viet subsets missing many characters

Extended Latin (latin-ext) and Vietnamese subsets are missing many characters. This directly affects Andika, Charis SIL, and Gentium Plus, but would affect any font family that has rich extended Latin support.

This was reported in July but remains a significant problem. Issue https://github.com/google/fonts/issues/3756

A large number of extended Latin and Vietnamese characters are not displaying properly. These characters are being displayed with fallback fonts even if the characters are supported in the fonts.

In the following screenshots LPR = local path-referenced font, GF = Google Font with `subset=latin-ext,cyrillic-ext,vietnamese`, FLO = our own internal font server. Screen shots are from current Chrome on Win 10.

Three specific examples:

1) Vietnamese text properly renders the Vietnamese diacritic forms when lang='vi' is set. However certain combinations with dot below are using fallback fonts.

Character string in example: Ấấ Ầầ Ẩẩ Ẫẫ Ắắ Ằằ Ẳẳ Ẵẵ Ếế Ềề Ểể Ễễ Ốố Ồồ Ổổ Ỗỗ Phải áp dụng chế độ giáo dục miễn phí, ít nhất là ở bậc tiểu học và giáo dục cơ sở

2) Extended Latin does not seem to include some important diacritics, such as U+0329, and again fallback fonts are used. Example from Yoruba language UDHR.

Character string in example: E̩nì kò̩ò̩kan ló ní è̩tó̩ láti kó̩ è̩kó̩. Ó kéré tán, è̩kó̩ gbo̩dò̩ jé̩ ò̩fé̩ ní àwo̩n ilé‐è̩kó̩ alákò̩ó̩bè̩rè̩. E̩kó̩ ní ilé‐è̩kó̩ alákò̩ó̩bè̩rè̩ yìí sì gbo̩dò̩ jé̩ dandan. A gbo̩dò̩ pèsè è̩kó̩ is̩é̩‐o̩wó̩, àti ti ìmò̩‐è̩ro̩ fún àwo̩n ènìyàn lápapò̩. Àn fàní tó dó̩gba ní ilé‐è̩kó̩ gíga gbo̩dò̩ wà ní àró̩wó̩tó gbogbo e̩ni tó bá tó̩ sí.

3) Many common diacritics, like ogonek, are not displaying properly

Character string in example: ọ o̧ ǫ ô o˞ o̝̠̣ ô͑ n f i fi f l fl ˥ ˦ ˧ ˨ ˩ ˥˥ ˥˦ ˥˧ ˥˨ ˥˩ ˥˨˥ ˥˨˦ ˥˨˧ ˥˨˨ ˥˨˩