update VarData.VarRegionCount
also set StartCoord and EndCoord to 0 (same end result as only setting PeakCoord
to 0, but this produces less noise when inspeciting the generated XML dump)
I added an MVAR table to the PartialInstancer-VF.ttx test font with made-up deltas for
OS/2.yStrikeoutSize, post.underlinePosition and post.underlineThickness.
I defined 3 regions, one with only wght, one with only wdth, and one with both wdth and wght axes.
