Glyphset coverage calculation: Check for *shapable* character rather than relying on cmap table

OT specs theoretically allow character composition on-the-fly from a base glyph and combining marks plus anchors without encoded characters being part of the font (`a`+`¨` is sufficient, `ä` needn't be present).

Glyphset coverage calculations currently rely on encoded characters to be present, but I'd like to widen the calculation to check for characters that are _shapable_, not necessarily present in the `cmap` and `glyf` tables.