[Indic] In final reordering, find base, even if it disappeared

POS_BASE can disappear if base ligated backward.  Define base as last
with position not after base.

Fixes a few hundred of Sinhala failures with Iskoola Pota.

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

@@ -705,11 +705,8 @@ final_reordering_syllable (hb_buffer_t *buffer,
 
   /* Find base again */
   unsigned int base = end;
-  for (unsigned int i = start; i < end; i++)
-    if (info[i].indic_position() == POS_BASE_C) {
-      base = i;
-      break;
-    }
+  while (start < base && info[base - 1].indic_position() >= POS_BASE_C)
+    base--;
 
   unsigned int start_of_last_cluster = base;
 

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

@@ -4,3 +4,4 @@
 කේ
 කෝ
 කෝ
+ස්ට්‍රේ