Add old-Myanmar shaper

Looks like Unsicribe responds to the 'mymr' tag by zeroing marks
GDEF_LATE instead of generic-shaper UNICODE_LATE.  Implement that.

Fixes
Bug 81775 - Incorrect Rendering with harfbuzz-ng myanmar unicode
https://bugs.freedesktop.org/show_bug.cgi?id=81775

Micro-test added based on Padauk.

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

diff --git a/src/hb-ot-shape-complex-myanmar.cc b/src/hb-ot-shape-complex-myanmar.cc
index 258ccc4..d016380 100644
--- a/src/hb-ot-shape-complex-myanmar.cc
+++ b/src/hb-ot-shape-complex-myanmar.cc
@@ -536,6 +536,24 @@
 }
 
 
+/* Uniscribe seems to have a shaper for 'mymr' that is like the
+ * generic shaper, except that it zeros mark advances GDEF_LATE. */
+const hb_ot_complex_shaper_t _hb_ot_complex_shaper_myanmar_old =
+{
+  "default",
+  NULL, /* collect_features */
+  NULL, /* override_features */
+  NULL, /* data_create */
+  NULL, /* data_destroy */
+  NULL, /* preprocess_text */
+  HB_OT_SHAPE_NORMALIZATION_MODE_DEFAULT,
+  NULL, /* decompose */
+  NULL, /* compose */
+  NULL, /* setup_masks */
+  HB_OT_SHAPE_ZERO_WIDTH_MARKS_BY_GDEF_LATE,
+  true, /* fallback_position */
+};
+
 const hb_ot_complex_shaper_t _hb_ot_complex_shaper_myanmar =
 {
   "myanmar",

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

diff --git a/src/hb-ot-shape-complex-private.hh b/src/hb-ot-shape-complex-private.hh
index 3e581af..9a7afcb 100644
--- a/src/hb-ot-shape-complex-private.hh
+++ b/src/hb-ot-shape-complex-private.hh
@@ -56,6 +56,7 @@
   HB_COMPLEX_SHAPER_IMPLEMENT (arabic) \
   HB_COMPLEX_SHAPER_IMPLEMENT (hangul) \
   HB_COMPLEX_SHAPER_IMPLEMENT (hebrew) \
+  HB_COMPLEX_SHAPER_IMPLEMENT (myanmar_old) \
   HB_COMPLEX_SHAPER_IMPLEMENT (indic) \
   HB_COMPLEX_SHAPER_IMPLEMENT (myanmar) \
   HB_COMPLEX_SHAPER_IMPLEMENT (sea) \
@@ -329,10 +330,10 @@
 	return &_hb_ot_complex_shaper_default;
 
     case HB_SCRIPT_MYANMAR:
-      /* For Myanmar, we only want to use the Myanmar shaper if the "new" script
-       * tag is found.  For "old" script tag we want to use the default shaper. */
       if (planner->map.chosen_script[0] == HB_TAG ('m','y','m','2'))
 	return &_hb_ot_complex_shaper_myanmar;
+      else if (planner->map.chosen_script[0] == HB_TAG ('m','y','m','r'))
+	return &_hb_ot_complex_shaper_myanmar_old;
       else
 	return &_hb_ot_complex_shaper_default;