Families should reserve vertical space for stacked diacritics (even before having such glyphs)

[felipesanches]

Stacked diacritics are common on scripts like Vietnamese

New font families should have reserved vertical space for future glyph coverage expansion into scripts that would require stacked diacritics, otherwise, it would be tricky to do so without breaking vertical metrics on documents typeset with previous releases of the font files.

I am not sure which heuristics to use for such a check, but I'd like to invite the community to propose reasonable approaches. My first thought about this topic yesterday was that the required extra vertical space could be the height of Agrave plus the height of gravecomb (effectively, that would be approximately the height of an uppercase A plus twice the height of a commonly present diacritic). But @tphinney suggested that we might not need that much perhaps.

On a second thought, though, @tphinney also considered (if I recall it correctly) that the Vietnamese hook is relatively tall...

@davelab6, @vv-monsalve, @RosaWagner, @m4rc1e, @twardoch and others, please let me know your suggestions here.