Add clarification about how multi-segment line glyphs are intended to be used

[dspreadbury]

The multi-segment glyphs in SMuFL are horizontally orientated at least partly for historical reasons, i.e. it’s how previous generations of music fonts have worked (though I guess that may not be the case for Emmentaler – I did look at Emmentaler in some detail early in the SMuFL project, but primarily for the purposes of surveying its glyph repertoire rather than examining its registration/orientation), and because it’s easy to use the normal advance width mechanism of typical left-to-right text layout to produce the appropriate tessellation. In general rotation of a run of text is easy (or easy-ish) to achieve regardless of the underlying graphics API.

SMuFL does encode a “repeatOffset” anchor in the metadata to allow apps that don’t render multi-segment lines as a simple run of text to know where to position the next item, but this is typically not needed if the anchor position matches the advance width of the character.

We probably should add some text to the specification providing some more insight about how these ranges are intended to be used.