In hb-coretext, when we were using scratch buffer for book-keeping,
a reverse_range() caused by the notdef-insertion loop could mess up
our log_clusters.  Ouch!

Fixes https://github.com/behdad/harfbuzz/pull/88

Before, the IntType::cmp functions providing this and was truncating
the hb_codepoint_t to 16bits before comparison.  I have no idea how
this was never discovered, and I'm too lazy to try to reproduce this
with Pango (which uses non-16bit codepoint numbers for missing glyphs).

I was hoping to see a nice speedup, but it resulted in a very minor one.

Shouldn't be needed.  I have a hard time imagining this breaking any
legitimate use case.

Needed some rework of Extension table.  Hopefully I got it right, and
the new template usage doesn't break any compilers...

No functional change right now.

This makes a lot of code safer.  We only try modifying the object in one
place, after making sure it's safe to do so.  So, do a const_cast<> in
that one place...

Can be further optimized, but I think I didn't break anything.

Saves another 3% off Roboto shaping.

Towards reducing the cost of initializing skippy_iter()

Has no functional effect since reject was never used with
match_glyph_data.