[Indic] Recategorize Khmer coeng sign as a separate category OT_Coeng

Amend the syllable structure to allow a final subscripted consonant
(Coeng+C) and a final subscripted independent vowel (Coeng+V).
Fixes another 2k of Khmer failures.

src/hb-ot-shape-complex-indic-machine.rl

@@ -58,14 +58,15 @@ Coeng = 15;
 c = C | Ra;
 n = N N?;
 z = ZWJ|ZWNJ;
+h = H | Coeng;
 matra_group = (M | RS) N? H?;
 syllable_tail = SM? (VD VD?)?;
 place_holder = NBSP | DOTTEDCIRCLE;
 
 
-consonant_syllable =	(c.n? (H.z?|z.H))* c.n? A? (H.z? | matra_group*)? syllable_tail;
-vowel_syllable =	(Ra H)? V n? (z?.H.c | ZWJ.c)* matra_group* syllable_tail;
-standalone_cluster =	(Ra H)? place_holder n? (z? H c)* matra_group* syllable_tail;
+consonant_syllable =	(c.n? (h.z?|z.h))* c.n? A? (h.z? | matra_group*)? (Coeng (c|V))? syllable_tail;
+vowel_syllable =	(Ra H)? V n? (z?.h.c | ZWJ.c)* matra_group* syllable_tail;
+standalone_cluster =	(Ra H)? place_holder n? (z? h c)* matra_group* syllable_tail;
 other =			any;
 
 main := |*

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

@@ -276,7 +276,11 @@ _hb_ot_shape_complex_setup_masks_indic (hb_ot_map_t *map HB_UNUSED,
 	unlikely (hb_in_range<hb_codepoint_t> (info.codepoint, 0x17CB, 0x17D0)))
       info.indic_category() = OT_RS;
 
-    if (info.indic_category() == OT_C) {
+    /* Khmer Virama is different since it can be used to form a final consonant. */
+    if (unlikely (info.codepoint == 0x17D2))
+      info.indic_category() = OT_Coeng;
+
+    if (is_consonant (info)) {
       info.indic_position() = consonant_position (info.codepoint);
       if (is_ra (info.codepoint))
 	info.indic_category() = OT_Ra;

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

@@ -7,3 +7,4 @@
 រី
 រ៍
 សៅ
+រ្ឥ