This resurrects the space fallback feature, after I disabled
the compatibility decomposition. Now I can release HarfBuzz
again without breaking Pango!
It also remembers which space character it was, such that later
on we can approximate the width of this particular space
character. That part is not implemented yet.
We normalize all GC=Zs chars except for U+1680 OGHA SPACE MARK,
which is better left alone.
Behdad Esfahbod