Bug 32274 - classic mongolian shaper

Add support for classic Mongolian script to the Arabic shaper.

Still work to be done around U+180E MONGOLIAN VOWEL SEPARATOR as it
should not be included in the final glyph stream the same way that
ZWNJ, etc should not appear in the final glyph stream.

But the joining part should be done.

There remains the question of how should the U+18A9 MONGOLIAN LETTER ALI
GALI DAGALGA be handled as it has General Category NSM but a letter
nonetheless.  For now, our generic logic makes this a joining T instead
of joining D as other Mongolian letters are.

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

@@ -67,6 +67,14 @@ static unsigned int get_joining_type (hb_codepoint_t u, hb_category_t gen_cat)
       return j_type;
   }
 
+  /* Mongolian joining data is not in ArabicJoining.txt yet */
+  if (unlikely (0x1800 <= u && u <= 0x18AF))
+  {
+    /* All letters, SIBE SYLLABLE BOUNDARY MARKER, and NIRUGU are D */
+    if (gen_cat == HB_CATEGORY_OTHER_LETTER || u == 0x1807 || u == 0x180A)
+      return JOINING_TYPE_D;
+  }
+
   if (unlikely ((u & ~(0x200C^0x200D)) == 0x200C)) {
     return u == 0x200C ? JOINING_TYPE_U : JOINING_TYPE_C;
   }

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

@@ -42,6 +42,7 @@ hb_ot_shape_complex_categorize (const hb_segment_properties_t *props)
     case HB_SCRIPT_NKO:
     case HB_SCRIPT_SYRIAC:
     case HB_SCRIPT_MANDAIC:
+    case HB_SCRIPT_MONGOLIAN:
       return hb_ot_complex_shaper_arabic;
 
     default: