Combining them, as originally envisioned, would lead to much more
complicated code, as investigations have shown meanwhile. The major
drawback is that we run out of available style slots much earlier.
However, this is only a theoretical issue since we don't support a
large number of scripts currently.
* src/autofit/afcover.h: Replace `COVERAGE_{1,2,3}' macros with
a single-element `COVERAGE' macro, sort the elements by the feature
tags, and add entry for `ruby'.
* src/autofit/aftypes.h: Updated.
* src/autofit/hbshim.c: Updated.
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