Fix base-position when 'pref' is NOT formed

If pre-base reordering Ra is NOT formed (or formed and then
broken up), we should consider that Ra as base.  This is
observable when there's a left matra or dotreph that positions
before base.

Now, it might be that we shouldn't do this if the Ra happend
to form a below form.  We can't quite deduce that right now...

Micro test added.  Also at:

https://code.google.com/a/google.com/p/noto-alpha/issues/detail?id=186#c29

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

@@ -1365,10 +1365,34 @@ final_reordering_syllable (const hb_ot_shape_plan_t *plan,
    * cluster.
    */
 
+  bool try_pref = !!indic_plan->mask_array[PREF];
+
   /* Find base again */
   unsigned int base;
   for (base = start; base < end; base++)
-    if (info[base].indic_position() >= POS_BASE_C) {
+    if (info[base].indic_position() >= POS_BASE_C)
+    {
+      if (try_pref && base + 1 < end && indic_plan->config->pref_len == 2)
+      {
+	for (unsigned int i = base + 1; i < end; i++)
+	  if ((info[i].mask & indic_plan->mask_array[PREF]) != 0)
+	  {
+	    if (!(_hb_glyph_info_substituted (&info[i]) &&
+		  _hb_glyph_info_ligated_and_didnt_multiply (&info[i])))
+	    {
+	      /* Ok, this was a 'pref' candidate but didn't form any.
+	       * Base is around here... */
+	      base = i;
+	      while (base < end && is_halant_or_coeng (info[base]))
+		base++;
+	      info[base].indic_position() = POS_BASE_C;
+
+	      try_pref = false;
+	    }
+	    break;
+	  }
+      }
+
       if (start < base && info[base].indic_position() > POS_BASE_C)
         base--;
       break;
@@ -1603,7 +1627,7 @@ final_reordering_syllable (const hb_ot_shape_plan_t *plan,
    *     the following rules:
    */
 
-  if (indic_plan->mask_array[PREF] && base + 1 < end) /* Otherwise there can't be any pre-base reordering Ra. */
+  if (try_pref && base + 1 < end) /* Otherwise there can't be any pre-base reordering Ra. */
   {
     unsigned int pref_len = indic_plan->config->pref_len;
     for (unsigned int i = base + 1; i < end; i++)

test/shaping/fonts/sha1sum/MANIFEST

+226bc2deab3846f1a682085f70c67d0421014144.ttf
+4cce528e99f600ed9c25a2b69e32eb94a03b4ae8.ttf
+d629e7fedc0b350222d7987345fe61613fa3929a.ttf
+e207635780b42f898d58654b65098763e340f5c7.ttf
+f499fbc23865022234775c43503bba2e63978fe1.ttf

test/shaping/tests/indic-pref-blocking.tests

-fonts/sha1sum/62927e416ab1fba8cc6222d97b2b0ba0e0eb00ea.ttf:U+0D2F,U+0D4D,U+0D30,U+0D46:[evowelsignmlym=0+1465|rapostmlym=0+499|yamlym=0+2120]
+fonts/sha1sum/226bc2deab3846f1a682085f70c67d0421014144.ttf:U+0D2F,U+0D4D,U+0D30,U+0D46:[evowelsignmlym=0+1465|rapostmlym=0+499|yamlym=0+2120]
+fonts/sha1sum/e207635780b42f898d58654b65098763e340f5c7.ttf:U+0D2F,U+0D4D,U+0D30,U+0D46:[yamlym=0+2120|viramamlym=0+0|evowelsignmlym=0+1465|ramlym=0+1507]