Make it possible to disable the fallback shaper at configure time

The OT shaper supersedes the fallback shaper in every case
and the latter become an extra weight for 99.9% of users.
Micro optimizations
Fix double destruction in case of OOM
[coretext] Minor optimization
[coretext] Handle surrogate pairs when generating notdef glyphs

Fixes github.com/behdad/harfbuzz/pull/19