WARN if no mark_to_mark feature

top comb accents can’t be combined on top of each other if they only have a _top anchors. They also need a top one.

I’ll update here with a proper test case when I find one.

[RosaWagner]

• Some fonts are lacking a second anchor (top) in comb accents (which goes on top of letters)

There are like this:

and we want this:

• But before that, some fonts are actually lacking all anchors:

And not only in comb-accents, also in letters. My question was; is this the same check?

Because missing first anchor is severity 8, missing second anchor is severity 6

[RosaWagner]

Looking more in detail in the GPOS table, I realise anchors name are not reported, there are only indexes. So to make it simpler:

For these common top marks in GF_Latin_* glyphsets: [uni0308, uni0307, gravecomb, acutecomb, uni030B, uni0302, uni030C, uni0306, uni030A, tildecomb, uni030F, uni0311, uni0312, uni0313]

(dieresiscomb, dotaccentcomb, gravecomb, acutecomb, hungarumlautcomb, circumflexcomb, caroncomb, brevecomb, ringcomb, tildecomb, macroncomb, dblgravecomb, breveinvertedcomb, commaturnedabovecomb, commaabovecomb)

WARN if they don’t allow a mark to mark positioning.

In the GPOS table it translates like this (I think):

<Mark1Coverage>
    <Glyph value="gravecomb"/>
    <Glyph value="acutecomb"/>
</Mark1Coverage>
<Mark2Coverage>
    <Glyph value="gravecomb"/>
    <Glyph value="acutecomb"/>
</Mark2Coverage>

Rationale: Mark to Mark feature is essential to allow user to stack diacritics together. Indeed many double accented glyphs don’t have any unicode codepoint, and the only way to type them properly is to enable that feature. Make sure the top combining marks has a _top anchor to get attached to base letter, and a top anchor above the accent’s outline to attach another diacritic.