add support for glyph shape variation along the spacing axis

as described in the [original proposal by Frank Blokland](http://github.com/Microsoft/OpenTypeDesignVariationAxisTags/blob/master/Proposals/Spacing_Axis/ProposalSummary.md):

> Glyph side-bearing distances (with ensuing effect on advance widths) are typically the primary aspect of design that varies, **though details such as serifs or spacing within ligatures may also be encompassed in this variation**.

how to implement glyph shape variation in the current VariableSpacing workflow:

- store glyph variations in optional additional layers – `SPAC_tight` , `SPAC_loose`
- switch glyphs when loading spacing state

open questions:

- how to express glyph shape variation in the [SpacingStates format](https://hipertipo.gitlab.io/VariableSpacing/reference/spacing-states-format/)?
- where to store the default glyph when another one is loaded? `SPAC_default`