[arabic] Disable fallback shaping if HB_NO_OT_SHAPE_COMPLEX_ARABIC_FALLBACK defined

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

src/hb-ot-shape-complex-arabic-fallback.hh

@@ -91,7 +91,6 @@ arabic_fallback_synthesize_lookup_single (const hb_ot_shape_plan_t *plan HB_UNUS
 				       hb_sorted_array (glyphs, num_glyphs),
 				       hb_array (substitutes, num_glyphs));
   c.end_serialize ();
-  /* TODO sanitize the results? */
 
   return ret ? c.copy<OT::SubstLookup> () : nullptr;
 }

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

@@ -383,6 +383,10 @@ arabic_fallback_shape (const hb_ot_shape_plan_t *plan,
 		       hb_font_t *font,
 		       hb_buffer_t *buffer)
 {
+#if defined(HB_NO_OT_SHAPE_COMPLEX_ARABIC_FALLBACK)
+  return;
+#endif
+
   const arabic_shape_plan_t *arabic_plan = (const arabic_shape_plan_t *) plan->data;
 
   if (!arabic_plan->do_fallback)