In MarkBase attachment, only attach to first of a MultipleSubst sequence

This is apparently what Uniscribe does.  Test case is:

  SEEN FATHA TEH ALEF

with Arabic Typesetting.  Originally reported by Khaled Hosny.

src/hb-ot-layout-gpos-table.hh

@@ -955,7 +955,12 @@ struct MarkBasePosFormat1
     /* now we search backwards for a non-mark glyph */
     unsigned int property;
     hb_apply_context_t::mark_skipping_backward_iterator_t skippy_iter (c, c->buffer->idx, 1);
-    if (!skippy_iter.prev (&property, LookupFlag::IgnoreMarks)) return TRACE_RETURN (false);
+    do {
+      if (!skippy_iter.prev (&property, LookupFlag::IgnoreMarks)) return TRACE_RETURN (false);
+      /* We only want to attach to the first of a MultipleSubst sequence.  Reject others. */
+      if (0 == get_lig_comp (c->buffer->info[skippy_iter.idx])) break;
+      skippy_iter.reject ();
+    } while (1);
 
     /* The following assertion is too strong, so we've disabled it. */
     if (!(property & HB_OT_LAYOUT_GLYPH_CLASS_BASE_GLYPH)) {/*return TRACE_RETURN (false);*/}

src/hb-ot-layout-gsubgpos-private.hh

@@ -144,6 +144,10 @@ struct hb_apply_context_t
     {
       return unlikely (num_items && idx + num_items >= end);
     }
+    inline bool reject (void)
+    {
+      num_items++;
+    }
     inline bool next (unsigned int *property_out,
 		      unsigned int lookup_props)
     {
@@ -189,6 +193,10 @@ struct hb_apply_context_t
     {
       return unlikely (idx < num_items);
     }
+    inline bool reject (void)
+    {
+      num_items++;
+    }
     inline bool prev (unsigned int *property_out,
 		      unsigned int lookup_props)
     {