[`GVAR`] Change `GlyphVariationData` Header `dataOffset` field to 24bit

[behdad]

In the GVAR equivalent of this gvar section:
https://learn.microsoft.com/en-us/typography/opentype/spec/gvar#the-glyphvariationdata-table-array

We've seen actual font builds hit the 64kb limit of that dataOffset field. Bumping to 24bits removes one of the tightest gvar limitations, perhaps the only practical one.