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

[yanone]

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.

[khaledhosny]

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).

Though this works in Chrome, in most applications if the pre-composed character is missing from the font, the application will use a fallback font and will not try the decomposed form first.