[lookup] Don't initialize skippy if coverage match fails

Currently:

  - Initializing skippy is very expensive,

  - Our lookup accelerator (using set-digests) can be very ineffecite,

As such, we end up many times initializing skippy but then failing
coverage check.  Reordering fixes that.

When, later, we fix our accelerator to have truly small false-positive
rate (for example by using the frozen-sets), then we might want to
reorder these checks such that we wouldn't calculate coverage number
if skippy is going to fail.

This shows a 5% speedup with Roboto already.

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

@@ -681,12 +681,11 @@ struct PairPosFormat1
   {
     TRACE_APPLY (this);
     hb_buffer_t *buffer = c->buffer;
-    hb_apply_context_t::skipping_forward_iterator_t skippy_iter (c, buffer->idx, 1);
-    if (skippy_iter.has_no_chance ()) return TRACE_RETURN (false);
-
     unsigned int index = (this+coverage).get_coverage  (buffer->cur().codepoint);
     if (likely (index == NOT_COVERED)) return TRACE_RETURN (false);
 
+    hb_apply_context_t::skipping_forward_iterator_t skippy_iter (c, buffer->idx, 1);
+    if (skippy_iter.has_no_chance ()) return TRACE_RETURN (false);
     if (!skippy_iter.next ()) return TRACE_RETURN (false);
 
     return TRACE_RETURN ((this+pairSet[index]).apply (c, &valueFormat1, skippy_iter.idx));
@@ -752,12 +751,11 @@ struct PairPosFormat2
   {
     TRACE_APPLY (this);
     hb_buffer_t *buffer = c->buffer;
-    hb_apply_context_t::skipping_forward_iterator_t skippy_iter (c, buffer->idx, 1);
-    if (skippy_iter.has_no_chance ()) return TRACE_RETURN (false);
-
     unsigned int index = (this+coverage).get_coverage  (buffer->cur().codepoint);
     if (likely (index == NOT_COVERED)) return TRACE_RETURN (false);
 
+    hb_apply_context_t::skipping_forward_iterator_t skippy_iter (c, buffer->idx, 1);
+    if (skippy_iter.has_no_chance ()) return TRACE_RETURN (false);
     if (!skippy_iter.next ()) return TRACE_RETURN (false);
 
     unsigned int len1 = valueFormat1.get_len ();
@@ -903,12 +901,11 @@ struct CursivePosFormat1
     /* We don't handle mark glyphs here. */
     if (unlikely (_hb_glyph_info_is_mark (&buffer->cur()))) return TRACE_RETURN (false);
 
-    hb_apply_context_t::skipping_forward_iterator_t skippy_iter (c, buffer->idx, 1);
-    if (skippy_iter.has_no_chance ()) return TRACE_RETURN (false);
-
     const EntryExitRecord &this_record = entryExitRecord[(this+coverage).get_coverage  (buffer->cur().codepoint)];
     if (!this_record.exitAnchor) return TRACE_RETURN (false);
 
+    hb_apply_context_t::skipping_forward_iterator_t skippy_iter (c, buffer->idx, 1);
+    if (skippy_iter.has_no_chance ()) return TRACE_RETURN (false);
     if (!skippy_iter.next ()) return TRACE_RETURN (false);
 
     const EntryExitRecord &next_record = entryExitRecord[(this+coverage).get_coverage  (buffer->info[skippy_iter.idx].codepoint)];