Add HB_TAG_MAX_SIGNED / _HB_SCRIPT_MAX_VALUE_SIGNED

To make C language police happy.

src/hb-common.h

@@ -95,6 +95,7 @@ typedef uint32_t hb_tag_t;
 
 #define HB_TAG_NONE HB_TAG(0,0,0,0)
 #define HB_TAG_MAX HB_TAG(0xff,0xff,0xff,0xff)
+#define HB_TAG_MAX_SIGNED HB_TAG(0x7f,0xff,0xff,0xff)
 
 /* len=-1 means str is NUL-terminated. */
 hb_tag_t
@@ -295,11 +296,17 @@ typedef enum
   /*7.0*/ HB_SCRIPT_WARANG_CITI			= HB_TAG ('W','a','r','a'),
 
   /* No script set. */
-  /*---*/ HB_SCRIPT_INVALID			= HB_TAG_NONE,
-
-  /* Dummy value to ensure any hb_tag_t value can be passed/stored as hb_script_t
-   * without risking undefined behavior. */
-  /*---*/ _HB_SCRIPT_MAX_VALUE			= HB_TAG_MAX /*< skip >*/
+  HB_SCRIPT_INVALID				= HB_TAG_NONE,
+
+  /* Dummy values to ensure any hb_tag_t value can be passed/stored as hb_script_t
+   * without risking undefined behavior.  Include both a signed and unsigned max,
+   * since technically enums are int, and indeed, hb_script_t ends up being signed.
+   * See this thread for technicalities:
+   *
+   *   http://lists.freedesktop.org/archives/harfbuzz/2014-March/004150.html
+   */
+  _HB_SCRIPT_MAX_VALUE				= HB_TAG_MAX, /*< skip >*/
+  _HB_SCRIPT_MAX_VALUE_SIGNED			= HB_TAG_MAX_SIGNED /*< skip >*/
 
 } hb_script_t;