Incorrect glyph advances using OT font funcs when HVAR is present

[jfkthame]

I'm seeing spacing problems with some variation fonts when using the OT font functions, whereas the FT functions give apparently-correct results.

Test font: AvenirNext_Variable.ttf pulled from http://www.axis-praxis.org/fonts/webfonts/AvenirNext_Variable.ttf.

Compare:

$ hb-shape --font-funcs ft AvenirNext_Variable.ttf Avenir
[gid36=0+694|gid89=1+488|gid72=2+572|gid81=3+581|gid76=4+250|gid85=5+360]

$ hb-shape --font-funcs ot AvenirNext_Variable.ttf Avenir
[gid36=0+692|gid89=1+636|gid72=2+563|gid81=3+656|gid76=4+370|gid85=5+462]

Looking at them with hb-view, the ft version is the correct one.

[jfkthame]

OTOH, if I explicitly set the 'wdth' coordinate, then I get what look like correct advances from the OT functions:

$ hb-shape --font-funcs ft --variations 'wdth'=100 AvenirNext_Variable.ttf Avenir
[gid36=0+694|gid89=1+488|gid72=2+572|gid81=3+581|gid76=4+250|gid85=5+360]

$ hb-shape --font-funcs ot --variations 'wdth'=100 AvenirNext_Variable.ttf Avenir
[gid36=0+694|gid89=1+488|gid72=2+572|gid81=3+581|gid76=4+250|gid85=5+360]

And with a non-default 'wdth' value, it's the OT version that looks correct, and the FT functions give me bad results:

$ hb-shape --font-funcs ft --variations 'wdth'=75 AvenirNext_Variable.ttf Avenir
[gid36=0+222|gid89=1+375|gid72=2+423|gid81=3+470|gid76=4+220|gid85=5+274]

$ hb-shape --font-funcs ot --variations 'wdth'=75 AvenirNext_Variable.ttf Avenir
[gid36=0+458|gid89=1+375|gid72=2+423|gid81=3+470|gid76=4+220|gid85=5+274]

The initial 'A' in the FT version has a much-too-small advance, although the other glyphs look OK.