The solution we had before worked in most cases but there were some
which caused problems still. This is what HarfBuzz's CoreText shaper
backend does, it uses a CTTypesetter with the forced embedding level
attribute. This fixes the failure case I found that was causing non-
monotonic outputs which can have all sorts of unexpected results, and
causes a crash in Debug modes because we assert the monotonicity while
rendering.
Variable font init used to just select the first available predefined
instance, if there were any, which is often not desirable- using
createFontDescriptorFromData instead of createFontDescritorsFromData
ensures that the default variation config is selected. In the future we
should probably allow selection of predefined instances, but for now
this is the correct behavior.
I found this bug when adding the metrics calculation test case for
CoreText, hence why fixing it is part of the same commit.
