[aat] Tweak fallback positioning logic when applying morx

Such that for Indic-like scripts (eg. Khmer), we don't do any fallback mark
advance-zeroing / positioning, but we do for Latin, etc.  Reuses preferences
of our script-specific OpenType shapers for those.

Fixes regression: https://github.com/harfbuzz/harfbuzz/issues/1393
Which means, fixes again: https://github.com/harfbuzz/harfbuzz/issues/1264
While not regressing: https://github.com/harfbuzz/harfbuzz/issues/1357

src/hb-ot-shape.hh

diff --git a/src/hb-ot-shape.hh b/src/hb-ot-shape.hh
index 397634c..6e1478d 100644
--- a/src/hb-ot-shape.hh
+++ b/src/hb-ot-shape.hh
@@ -77,6 +77,7 @@
   bool zero_marks : 1;
   bool fallback_glyph_classes : 1;
   bool fallback_mark_positioning : 1;
+  bool adjust_mark_positioning_when_zeroing : 1;
 
   bool apply_gpos : 1;
   bool apply_kerx : 1;
@@ -113,6 +114,8 @@
   hb_ot_map_builder_t map;
   hb_aat_map_builder_t aat_map;
   bool apply_morx : 1;
+  bool script_zero_marks : 1;
+  bool script_fallback_mark_positioning : 1;
   const struct hb_ot_complex_shaper_t *shaper;
 
   HB_INTERNAL hb_ot_shape_planner_t (hb_face_t                     *face,