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",