[Indic] Implement Reph+Ya-Phalaa interaction

The sequence Ra,H,Ya in Bengali is ambigious and Unicode encoded that to
get Ya-Phalaa, one would place ZWJ before Halant.  Ie. a ZWJ,H sequence
requests subjoining, while a H,ZWJ requests Half form.  Implement that.

Bengali failures go down from 377 to 297 (0.0838308%).
Gujarati is down by 4 to 17 (0.0046384%).
Kannada is down by 226 to 957 (0.100534%).

Current status:

BENGALI: 353988 out of 354285 tests passed. 297 failed (0.0838308%)
DEVANAGARI: 693571 out of 693628 tests passed. 57 failed (0.00821766%)
GUJARATI: 366489 out of 366506 tests passed. 17 failed (0.0046384%)
GURMUKHI: 60750 out of 60809 tests passed. 59 failed (0.0970251%)
KANNADA: 950956 out of 951913 tests passed. 957 failed (0.100534%)
KHMER: 299094 out of 299124 tests passed. 30 failed (0.0100293%)
MALAYALAM: 1046857 out of 1048416 tests passed. 1559 failed (0.148701%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271699 out of 271847 tests passed. 148 failed (0.0544424%)
TAMIL: 1091837 out of 1091837 tests passed. 0 failed (0%)
TELUGU: 970524 out of 970573 tests passed. 49 failed (0.00504856%)

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

@@ -553,8 +553,14 @@ initial_reordering_consonant_syllable (const hb_ot_map_t *map, hb_buffer_t *buff
 	}
 	else
 	{
-	  /* A ZWJ stops the base search, and requests an explicit half form. */
-	  if (info[i].indic_category() == OT_ZWJ)
+	  /* A ZWJ after a Halant stops the base search, and requests an explicit
+	   * half form.
+	   * A ZWJ before a Halant, requests a subjoined form instead, and hence
+	   * search continues.  This is particularly important for Bengali
+	   * sequence Ra,H,Ya that shouls form Ya-Phalaa by subjoining Ya. */
+	  if (start < i &&
+	      info[i].indic_category() == OT_ZWJ &&
+	      info[i - 1].indic_category() == OT_H)
 	    break;
 	}
       } while (i > limit);

test/shaping/texts/in-tree/shaper-indic/indic/script-bengali/misc/reph.txt

@@ -8,3 +8,7 @@
 র্কৈ
 র্কো
 র্কৌ
+র্য
+র্‍য
+র‍্য
+র্র‍্য