Reduce max nesting level from 8 to 6

We probably should implement better system to catch cyclic lookups.
But for now, this speeds up worst case behavior with broken fonts
considerably without compromising legitimate usecases.

https://github.com/behdad/harfbuzz/issues/139#issuecomment-147788447

src/hb-ot-layout-common-private.hh

@@ -44,7 +44,7 @@ namespace OT {
 
 
 #define NOT_COVERED		((unsigned int) -1)
-#define MAX_NESTING_LEVEL	8
+#define MAX_NESTING_LEVEL	6
 #define MAX_CONTEXT_LENGTH	64