[HB] Tweak some constants and fix Coverage
diff --git a/src/hb-ot-layout-open-private.h b/src/hb-ot-layout-open-private.h
index 0dd291a..98a2499 100644
--- a/src/hb-ot-layout-open-private.h
+++ b/src/hb-ot-layout-open-private.h
@@ -35,9 +35,9 @@
#define NO_INDEX ((unsigned int) 0xFFFF)
-#define NO_CONTEXT ((unsigned int) -0x20000)
-#define NOT_COVERED ((unsigned int) -1)
-#define MAX_NESTING_LEVEL 32
+#define NO_CONTEXT ((unsigned int) 0x110000)
+#define NOT_COVERED ((unsigned int) 0x110000)
+#define MAX_NESTING_LEVEL 8
/*