[Indic] Recognizer Register Shifter marks

Fixes another 6% of the Khmer failures.

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

@@ -52,11 +52,12 @@ VD   = 10;
 A    = 11;
 NBSP = 12;
 DOTTEDCIRCLE = 13;
+RS   = 14;
 
 c = C | Ra;
 n = N N?;
 z = ZWJ|ZWNJ;
-matra_group = M N? H?;
+matra_group = (M | RS) N? H?;
 syllable_tail = SM? (VD VD?)?;
 place_holder = NBSP | DOTTEDCIRCLE;
 

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

@@ -58,7 +58,8 @@ enum indic_category_t {
   OT_VD,
   OT_A,
   OT_NBSP,
-  OT_DOTTEDCIRCLE /* Not in the spec, but special in Uniscribe. /Very very/ special! */
+  OT_DOTTEDCIRCLE, /* Not in the spec, but special in Uniscribe. /Very very/ special! */
+  OT_RS /* Register Shifter, used in Khmer OT spec */
 };
 
 /* Visual positions in a syllable from left to right. */
@@ -92,7 +93,7 @@ enum indic_syllabic_category_t {
   INDIC_SYLLABIC_CATEGORY_CONSONANT_REPHA	= OT_C,
   INDIC_SYLLABIC_CATEGORY_MODIFYING_LETTER	= OT_X,
   INDIC_SYLLABIC_CATEGORY_NUKTA			= OT_N,
-  INDIC_SYLLABIC_CATEGORY_REGISTER_SHIFTER	= OT_X,
+  INDIC_SYLLABIC_CATEGORY_REGISTER_SHIFTER	= OT_RS,
   INDIC_SYLLABIC_CATEGORY_TONE_LETTER		= OT_X,
   INDIC_SYLLABIC_CATEGORY_TONE_MARK		= OT_X,
   INDIC_SYLLABIC_CATEGORY_VIRAMA		= OT_H,

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

@@ -261,6 +261,8 @@ _hb_ot_shape_complex_setup_masks_indic (hb_ot_map_t *map HB_UNUSED,
       info.indic_position() = consonant_position (info.codepoint);
       if (is_ra (info.codepoint))
 	info.indic_category() = OT_Ra;
+    } else if (info.indic_category() == OT_RS) {
+      info.indic_position() = POS_ABOVE_M;
     } else if (info.indic_category() == OT_SM ||
 	       info.indic_category() == OT_VD) {
       info.indic_position() = POS_SMVD;