Add support for tuple variation store

We want to support variable glyphs and the [tuple variation store](https://learn.microsoft.com/en-us/typography/opentype/spec/otvarcommonformats#tuple-variation-store) is currently missing from `read-fonts`. This is needed for the [gvar](https://learn.microsoft.com/en-us/typography/opentype/spec/gvar) and [cvar](https://learn.microsoft.com/en-us/typography/opentype/spec/cvar) to enable computing deltas for glyph outlines and control values, respectively.

Some hand written code for this is available [here](https://github.com/dfrg/pinot/blob/59db153ff83420449a909dfaace18466bddbf814/moscato/src/truetype/var.rs) for reference.

Start in the read-fonts implementation and read the OpenType tuple variation store specification, then compare the reference code in moscato/src/truetype/var.rs. Trace how gvar and cvar need the store to compute deltas. Done means read-fonts supports the tuple variation store needed by both tables.

Written by the indexing model from the issue text.