[GSUB/GPOS] Use Coverage digests as gatekeeper

Gives me a good 10% speedup for the Devanagari test case.  Less so
for less lookup-intensive tests.

For the Devanagari test case, the false positive rate of the GSUB digest
is 4%.

src/hb-ot-layout-private.hh

diff --git a/src/hb-ot-layout-private.hh b/src/hb-ot-layout-private.hh
index fdbeb5b..d87a138 100644
--- a/src/hb-ot-layout-private.hh
+++ b/src/hb-ot-layout-private.hh
@@ -35,6 +35,7 @@
 
 #include "hb-font-private.hh"
 #include "hb-buffer-private.hh"
+#include "hb-set-private.hh"
 
 
 /* buffer var allocations, used during the GSUB/GPOS processing */
@@ -168,6 +169,9 @@
   const struct GDEF *gdef;
   const struct GSUB *gsub;
   const struct GPOS *gpos;
+
+  hb_set_digest_t *gsub_digests;
+  hb_set_digest_t *gpos_digests;
 };