[Indic] Move syllable tracking from Indic to generic layer

This is to incorporate it into GSUB/GPOS processing.
diff --git a/src/hb-ot-layout-gsub-table.hh b/src/hb-ot-layout-gsub-table.hh
index 682303a..bc2b0e1 100644
--- a/src/hb-ot-layout-gsub-table.hh
+++ b/src/hb-ot-layout-gsub-table.hh
@@ -1169,10 +1169,11 @@
 {
   HB_BUFFER_ALLOCATE_VAR (buffer, props_cache);
   HB_BUFFER_ALLOCATE_VAR (buffer, lig_props);
+  HB_BUFFER_ALLOCATE_VAR (buffer, syllable);
 
   unsigned int count = buffer->len;
   for (unsigned int i = 0; i < count; i++)
-    buffer->info[i].props_cache() = buffer->info[i].lig_props() = 0;
+    buffer->info[i].props_cache() = buffer->info[i].lig_props() = buffer->info[i].syllable() = 0;
 }
 
 void