SVG animation: allow the unanimated axes to be at non-default locations

[Lorp]

This should not be difficult. But remember that animating multiple axes at once in general results in non-linear interpolation. However, if the tuples that affect an axis affect that axis and no other, then we retain piecewise linearity. Corner masters are a problem, so in a typical wght/wdth font with corner masters, the non-linearity issue means we cannot export faithful SVG animations of animating both axes at once.