Add prefer_decomposed() complex-shaper callback

This allows the Indic shaper to request decomposed characters.  This will
handle split matra for free.  Other shapers prefer precomposed
characters.

src/hb-ot-shape-complex-arabic.cc

@@ -183,6 +183,12 @@ _hb_ot_shape_complex_collect_features_arabic (hb_ot_map_builder_t *map, const hb
   map->add_bool_feature (HB_TAG('c','s','w','h'));
 }
 
+bool
+_hb_ot_shape_complex_prefer_decomposed_arabic (void)
+{
+  return FALSE;
+}
+
 void
 _hb_ot_shape_complex_setup_masks_arabic (hb_ot_map_t *map, hb_buffer_t *buffer)
 {

src/hb-ot-shape-complex-indic.cc

@@ -309,6 +309,13 @@ _hb_ot_shape_complex_collect_features_indic (hb_ot_map_builder_t *map, const hb_
 }
 
 
+bool
+_hb_ot_shape_complex_prefer_decomposed_indic (void)
+{
+  /* We want split matras decomposed by the common shaping logic. */
+  return TRUE;
+}
+
 static void
 found_syllable (hb_ot_map_t *map, hb_buffer_t *buffer,
 		unsigned int start, unsigned int end)

src/hb-ot-shape-complex-misc.cc

@@ -42,6 +42,12 @@ _hb_ot_shape_complex_collect_features_default (hb_ot_map_builder_t *map, const h
 {
 }
 
+bool
+_hb_ot_shape_complex_prefer_decomposed_default (void)
+{
+  return FALSE;
+}
+
 void
 _hb_ot_shape_complex_setup_masks_default (hb_ot_map_t *map, hb_buffer_t *buffer)
 {

src/hb-ot-shape-complex-private.hh

@@ -147,6 +147,33 @@ hb_ot_shape_complex_collect_features (hb_ot_complex_shaper_t shaper,
 }
 
 
+/*
+ * prefer_decomposed()
+ *
+ * Called during shape_execute().
+ *
+ * Shapers should return TRUE if it prefers decomposed (NFD) input rather than precomposed (NFC).
+ */
+
+typedef bool hb_ot_shape_complex_prefer_decomposed_func_t (void);
+#define HB_COMPLEX_SHAPER_IMPLEMENT(name) \
+  HB_INTERNAL hb_ot_shape_complex_prefer_decomposed_func_t _hb_ot_shape_complex_prefer_decomposed_##name;
+  HB_COMPLEX_SHAPERS_IMPLEMENT_SHAPERS
+#undef HB_COMPLEX_SHAPER_IMPLEMENT
+
+static inline bool
+hb_ot_shape_complex_prefer_decomposed (hb_ot_complex_shaper_t shaper)
+{
+  switch (shaper) {
+    default:
+#define HB_COMPLEX_SHAPER_IMPLEMENT(name) \
+    case hb_ot_complex_shaper_##name:	return _hb_ot_shape_complex_prefer_decomposed_##name ();
+    HB_COMPLEX_SHAPERS_IMPLEMENT_SHAPERS
+#undef HB_COMPLEX_SHAPER_IMPLEMENT
+  }
+}
+
+
 /* setup_masks()
  *
  * Called during shape_execute().