Allow HB_OPTIONS=aat to prefer AAT tables over OT

Fixes https://github.com/harfbuzz/harfbuzz/issues/322
diff --git a/src/hb-debug.hh b/src/hb-debug.hh
index 12b6c49..58c190d 100644
--- a/src/hb-debug.hh
+++ b/src/hb-debug.hh
@@ -43,9 +43,10 @@
 
 struct hb_options_t
 {
-  unsigned int unused : 1; /* In-case sign bit is here. */
-  unsigned int initialized : 1;
-  unsigned int uniscribe_bug_compatible : 1;
+  bool unused : 1; /* In-case sign bit is here. */
+  bool initialized : 1;
+  bool uniscribe_bug_compatible : 1;
+  bool aat : 1;
 };
 
 union hb_options_union_t {