OpenType TupleVariations can be used in two places:
* In the `gvar` table, they modify glyph contour points by shifting
them towards a point in 2D space.
* In the `cvar` table, they modify constant values.
Before this change, we only had code to handle the `gvar` version
which shifts points around by delta values.
After this change, the XML parsing and generation routines of
TupleVariations can handle deltas that modify constant values in the
CVT table, as used in `cvar`.
An upcoming change will add support for the binary encoding of
TupleVariations as needed for `cvar`.
