[aat] Adjust fallback positioning logic

Fixes https://github.com/harfbuzz/harfbuzz/issues/1528

Wish could add a test...

$ ./hb-view --font-file Thonburi.ttc --unicodes U+0E17,U+0E35,U+0E48,U+0E4A --shaper ot

                   ▃

         ▂▃▃▄▃▂▁▊        ▃  ▃       ▎
      ▗   ▅▆▆▅          ▌ ▆▆▅▆▙ ▌▗
     ▗                  ▙ ▂  ▎▗   ▟
     ▅▆▆▆▇▇▇▇▇▇▇▆▆▅▄     ▇▅▅▅ ▙▆▇
    ▁▁▂▁       ▁▁
         ▖                    ▖ ▂▂
  ▉   ▁        ▆▆        ▌   ▆  ▙▄ ▁▁
   ▙            ▉      ▃▖           ▟
      ▊         ▉      ▅▟           ▗▄
      ▊         ▉     ▗ ▏           ▇▆
      ▊    ▟    ▉      ▆             ▖
      ▊         ▉        ▌        ▂
      ▊   ▏     ▉           ▌ ▗   ▄▟
      ▊                        ▆

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

@@ -49,5 +49,25 @@ const hb_ot_complex_shaper_t _hb_ot_complex_shaper_default =
   true, /* fallback_position */
 };
 
+/* Same as default but no mark advance zeroing / fallback positioning.
+ * Dumbest shaper ever, basically. */
+const hb_ot_complex_shaper_t _hb_ot_complex_shaper_dumber =
+{
+  nullptr, /* collect_features */
+  nullptr, /* override_features */
+  nullptr, /* data_create */
+  nullptr, /* data_destroy */
+  nullptr, /* preprocess_text */
+  nullptr, /* postprocess_glyphs */
+  HB_OT_SHAPE_NORMALIZATION_MODE_DEFAULT,
+  nullptr, /* decompose */
+  nullptr, /* compose */
+  nullptr, /* setup_masks */
+  HB_TAG_NONE, /* gpos_tag */
+  nullptr, /* reorder_marks */
+  HB_OT_SHAPE_ZERO_WIDTH_MARKS_NONE,
+  false, /* fallback_position */
+};
+
 
 #endif

src/hb-ot-shape-complex.hh

@@ -50,8 +50,9 @@ enum hb_ot_shape_zero_width_marks_type_t {
 
 /* Master OT shaper list */
 #define HB_COMPLEX_SHAPERS_IMPLEMENT_SHAPERS \
-  HB_COMPLEX_SHAPER_IMPLEMENT (default) /* should be first */ \
   HB_COMPLEX_SHAPER_IMPLEMENT (arabic) \
+  HB_COMPLEX_SHAPER_IMPLEMENT (default) \
+  HB_COMPLEX_SHAPER_IMPLEMENT (dumber) \
   HB_COMPLEX_SHAPER_IMPLEMENT (hangul) \
   HB_COMPLEX_SHAPER_IMPLEMENT (hebrew) \
   HB_COMPLEX_SHAPER_IMPLEMENT (indic) \
@@ -60,7 +61,7 @@ enum hb_ot_shape_zero_width_marks_type_t {
   HB_COMPLEX_SHAPER_IMPLEMENT (myanmar_zawgyi) \
   HB_COMPLEX_SHAPER_IMPLEMENT (thai) \
   HB_COMPLEX_SHAPER_IMPLEMENT (use) \
-  /* ^--- Add new shapers here */
+  /* ^--- Add new shapers here; keep sorted. */
 
 
 struct hb_ot_complex_shaper_t

src/hb-ot-shape.cc

@@ -78,8 +78,9 @@ hb_ot_shape_planner_t::hb_ot_shape_planner_t (hb_face_t                     *fac
   script_zero_marks = shaper->zero_width_marks != HB_OT_SHAPE_ZERO_WIDTH_MARKS_NONE;
   script_fallback_mark_positioning = shaper->fallback_position;
 
-  if (apply_morx)
-    shaper = &_hb_ot_complex_shaper_default;
+  /* https://github.com/harfbuzz/harfbuzz/issues/1528 */
+  if (apply_morx && shaper != &_hb_ot_complex_shaper_default)
+    shaper = &_hb_ot_complex_shaper_dumber;
 }
 
 void