[Indic] Position final subjoined consonants (and vowels) after matras

In Khmer, a final subjoined consonant or independent vowel can occur
after matras.  This final subjoined thing should NOT be reordered to
before the matra even though it's subjoined.

Fixes another 1k of the Khmer failures.  Not much left really.

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

@@ -75,6 +75,7 @@ enum indic_position_t {
   POS_BELOW_M,
   POS_POST_C,
   POS_POST_M,
+  POS_FINAL_C,
   POS_SMVD
 };
 

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

@@ -429,8 +429,21 @@ initial_reordering_consonant_syllable (const hb_ot_map_t *map, hb_buffer_t *buff
 
   for (unsigned int i = start; i < base; i++)
     info[i].indic_position() = POS_PRE_C;
+
   info[base].indic_position() = POS_BASE_C;
 
+  /* Mark final consonants.  A final consonant is one appearing after a matra,
+   * like in Khmer. */
+  for (unsigned int i = base + 1; i < end; i++)
+    if (info[i].indic_category() == OT_M) {
+      for (unsigned int j = i + 1; j < end; j++)
+        if (is_consonant (info[j])) {
+	  info[j].indic_position() = POS_FINAL_C;
+	  break;
+	}
+      break;
+    }
+
   /* Handle beginning Ra */
   if (has_reph)
     info[start].indic_position() = POS_RA_TO_BECOME_REPH;

test/shaping/texts/in-tree/shaper-indic/south-east-asian/script-khmer/misc/misc.txt

@@ -8,3 +8,5 @@
 រ៍
 សៅ
 រ្ឥ
+ងឹ្ឈ
+ង្ឈឹ