[autofit] Speed up construction of reverse character map. (2/4)

This commit, together with the previous one, makes the startup of the
auto-hinter much faster.

Note, though, that the startup time for handling the diacritic database is
still rather large.  For example, loading `arial.ttf` version 7.00 followed
by auto-hinting a first Latin glyph is still approx. three times slower
than before the introduction of the database (this is because function
`hb_ot_layout_lookup_get_glyph_alternates` is called very often).

* src/autofit/afshaper.c (scripts): Rename this array to...
  (af_hb_scripts): ...this and export it.
  (af_shaper_get_coverage_hb): Updated.
* src/autofit/afshaper.h: Updated.

* src/autofit/afadjust.c (af_reverse_character_map_new): Pass the current
  script to `hb_ot_layout_collect_lookups` to make HarfBuzz check less
  lookups while searching for glyph alternates.

src/autofit/afadjust.c

@@ -1398,12 +1398,21 @@
       hb_set_t  *result_set   = hb( set_create )();
       hb_set_t  *gsub_lookups = hb( set_create )();
 
+      hb_script_t  script = af_hb_scripts[metrics->style_class->script];
+
+      unsigned int  script_count = 1;
+      hb_tag_t      script_tags[2];
 
 
-      /* Compute set of all GSUB lookups. */
+      hb( ot_tags_from_script_and_language )( script, NULL,
+                                              &script_count, script_tags,
+                                              NULL, NULL );
+      script_tags[1] = HB_TAG_NONE;
+
+      /* Compute set of all script-specific GSUB lookups. */
       hb( ot_layout_collect_lookups )( hb_face,
                                        HB_OT_TAG_GSUB,
-                                       NULL, NULL, NULL,
+                                       script_tags, NULL, NULL,
                                        gsub_lookups );
 
       /* Find all glyph alternates of the code points in  */

src/autofit/afshaper.c

@@ -89,7 +89,8 @@
 #define SCRIPT( s, S, d, h, H, ss )  h,
 
 
-  static const hb_script_t  scripts[] =
+  FT_LOCAL_ARRAY_DEF( hb_script_t )
+  af_hb_scripts[] =
   {
 #include "afscript.h"
   };
@@ -127,7 +128,7 @@
     face = hb( font_get_face )( globals->hb_font );
 
     coverage_tags = coverages[style_class->coverage];
-    script        = scripts[style_class->script];
+    script        = af_hb_scripts[style_class->script];
 
     /* Convert a HarfBuzz script tag into the corresponding OpenType */
     /* tag or tags -- some Indic scripts like Devanagari have an old */

src/autofit/afshaper.h

@@ -25,6 +25,12 @@
 
 FT_BEGIN_HEADER
 
+#ifdef FT_CONFIG_OPTION_USE_HARFBUZZ
+  FT_LOCAL_ARRAY( hb_script_t )
+  af_hb_scripts[];
+#endif
+
+
   FT_Error
   af_shaper_get_coverage( AF_FaceGlobals  globals,
                           AF_StyleClass   style_class,