[Indic] Implement Sinhala "Al Lakuna" Reph behavior

In Sinhala, Reph is formed only explicitly, by the presence of a ZWJ.

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

@@ -269,6 +269,8 @@ static const hb_codepoint_t ra_chars[] = {
   0x0CB0, /* Kannada */
   0x0D30, /* Malayalam */	/* No Reph, Logical Repha */
 
+  0x0DBB, /* Sinhala */		/* Reph formed only with ZWJ */
+
   0x179A, /* Khmer */		/* No Reph, Visual Repha */
 };
 

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

@@ -356,9 +356,14 @@ initial_reordering_consonant_syllable (const hb_ot_map_t *map, hb_buffer_t *buff
 	start + 3 <= end &&
 	info[start].indic_category() == OT_Ra &&
 	info[start + 1].indic_category() == OT_H &&
-	!is_joiner (info[start + 2]))
+	(unlikely (buffer->props.script == HB_SCRIPT_SINHALA) ?
+	 info[start + 2].indic_category() == OT_ZWJ /* In Sinhala, form Reph only if ZWJ is present */:
+	 !is_joiner (info[start + 2] /* In other scripts, any joiner blocks Reph formation */ )
+	))
     {
       limit += 2;
+      while (limit < end && is_joiner (info[limit]))
+        limit++;
       base = start;
       has_reph = true;
     };

test/shaping/texts/in-tree/shaper-indic/indic/script-sinhala/misc/MANIFEST

@@ -1 +1,2 @@
 misc.txt
+reph.txt

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

@@ -0,0 +1,3 @@
+ර්ධ
+ර්‍ධ
+ර්‌ධ