[Indic] Categorize other Khmer marks

Mark them the same as the Register Shifters for now.  Need to rename
that category to something more sensible after all is settled.

Fixes another percent of Khmer failures.  Down to under 3%!

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

@@ -257,6 +257,10 @@ _hb_ot_shape_complex_setup_masks_indic (hb_ot_map_t *map HB_UNUSED,
     if (unlikely (hb_in_range<hb_codepoint_t> (info.codepoint, 0x0951, 0x0954)))
       info.indic_category() = OT_VD;
 
+    if (info.indic_category() == OT_X &&
+	unlikely (hb_in_range<hb_codepoint_t> (info.codepoint, 0x17CB, 0x17D0)))
+      info.indic_category() = OT_RS;
+
     if (info.indic_category() == OT_C) {
       info.indic_position() = consonant_position (info.codepoint);
       if (is_ra (info.codepoint))