ME_InsertTextFromCursor could previously follow the ME_CommitUndo call
leaving uncommitted changes which would cause improper grouping of commits,
or an assertion failure.

ME_InsertTextFromCursor copies the string passed into it rather than making
taking ownership of it.

This also involves a restructuring of the handling of glyph lookups
from GSUB since Ligature Substitution can result in many glyphs being
replaced with 1 glyph, and future looksup may result in one glyph
being replaces with many glyphs.