[Indic] Disable 'kern' in Uniscribe bug compatibility mode

Uniscribe does not apply 'kern' in the Indic module.  Some of the Khmer
fonts they ship have small adjustments in the 'kern' table.  Disable
'kern' in the Indic module under Uniscribe bug compatibility mode.

Fixes some 10% of the Khmer failures.  Remains under 3% (excluding
dotted-circle ones).

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

@@ -221,6 +221,9 @@ void
 _hb_ot_shape_complex_override_features_indic (hb_ot_map_builder_t *map,
 					      const hb_segment_properties_t *props HB_UNUSED)
 {
+  /* Uniscribe does not apply 'kern'. */
+  if (indic_options ().uniscribe_bug_compatible)
+    map->add_feature (HB_TAG('k','e','r','n'), 0, true);
 }