[HarfBuzz] Context matching

[Behdad Esfahbod]

Hi!

It's about 7pm in Paris and Jonathan and I just pushed out the rewritten
"apply_lookup" to master.  This fixes the last remaining "TODO" item in the
GSUB/GPOS code, ie. (Chain)Context lookups with non-ascending sequence of
recursed lookups.  We've tested this and seems to work fine and NOT break
thing, though we never know!

This fixes rendering of certain sequences with Noto Bengali and Noto
Devanagari, among others.

We also added a tiny test suite infrastructure in place to keep adding minimal
tests as we fix things.

Khaled, Richard, others, please test!!

-- 
behdad
http://behdad.org/