[vector] Add internal API for exact-size allocation

Use it from a couple of places.

src/OT/glyf/SimpleGlyph.hh

@@ -184,7 +184,7 @@ struct SimpleGlyph
     if (unlikely (!bytes.check_range (&endPtsOfContours[num_contours]))) return false;
     unsigned int num_points = endPtsOfContours[num_contours - 1] + 1;
 
-    points_.alloc (num_points + 4); // Allocate for phantom points, to avoid a possible copy
+    points_.alloc (num_points + 4, true); // Allocate for phantom points, to avoid a possible copy
     if (!points_.resize (num_points)) return false;
     if (phantom_only) return true;
 
@@ -272,9 +272,9 @@ struct SimpleGlyph
     unsigned num_points = all_points.length - 4;
 
     hb_vector_t<uint8_t> flags, x_coords, y_coords;
-    if (unlikely (!flags.alloc (num_points))) return false;
-    if (unlikely (!x_coords.alloc (2*num_points))) return false;
-    if (unlikely (!y_coords.alloc (2*num_points))) return false;
+    if (unlikely (!flags.alloc (num_points, true))) return false;
+    if (unlikely (!x_coords.alloc (2*num_points, true))) return false;
+    if (unlikely (!y_coords.alloc (2*num_points, true))) return false;
 
     uint8_t lastflag = 255, repeat = 0;
     int prev_x = 0, prev_y = 0;

src/OT/glyf/glyf.hh

@@ -429,7 +429,7 @@ glyf::_create_font_for_instancing (const hb_subset_plan_t *plan) const
   if (unlikely (font == hb_font_get_empty ())) return nullptr;
 
   hb_vector_t<hb_variation_t> vars;
-  if (unlikely (!vars.alloc (plan->user_axes_location->get_population ())))
+  if (unlikely (!vars.alloc (plan->user_axes_location->get_population (), true)))
     return nullptr;
 
   for (auto _ : *plan->user_axes_location)

src/hb-ot-layout-gsubgpos.hh

@@ -4020,7 +4020,7 @@ struct hb_ot_layout_lookup_accelerator_t
   void init (const TLookup &lookup)
   {
     subtables.init ();
-    subtables.alloc (lookup.get_subtable_count ());
+    subtables.alloc (lookup.get_subtable_count (), true);
     hb_accelerate_subtables_context_t c_accelerate_subtables (subtables);
     lookup.dispatch (&c_accelerate_subtables);
 

src/hb-repacker.hh

@@ -115,7 +115,7 @@ bool _promote_extensions_if_needed (graph::gsubgpos_graph_context_t& ext_context
   if (!ext_context.lookups) return true;
 
   hb_vector_t<lookup_size_t> lookup_sizes;
-  lookup_sizes.alloc (ext_context.lookups.get_population ());
+  lookup_sizes.alloc (ext_context.lookups.get_population (), true);
 
   for (unsigned lookup_index : ext_context.lookups.keys ())
   {

src/hb-serialize.hh

@@ -81,11 +81,11 @@ struct hb_serialize_context_t
       head = o.head;
       tail = o.tail;
       next = nullptr;
-      real_links.alloc (o.num_real_links);
+      real_links.alloc (o.num_real_links, true);
       for (unsigned i = 0 ; i < o.num_real_links; i++)
         real_links.push (o.real_links[i]);
 
-      virtual_links.alloc (o.num_virtual_links);
+      virtual_links.alloc (o.num_virtual_links, true);
       for (unsigned i = 0; i < o.num_virtual_links; i++)
         virtual_links.push (o.virtual_links[i]);
     }

src/hb-vector.hh

@@ -337,7 +337,7 @@ struct hb_vector_t
   }
 
   /* Allocate for size but don't adjust length. */
-  bool alloc (unsigned int size)
+  bool alloc (unsigned int size, bool exact=false)
   {
     if (unlikely (in_error ()))
       return false;
@@ -347,8 +347,8 @@ struct hb_vector_t
 
     /* Reallocate */
 
-    unsigned int new_allocated = allocated;
-    while (size >= new_allocated)
+    unsigned int new_allocated = exact ? size : allocated;
+    while (size > new_allocated)
       new_allocated += (new_allocated >> 1) + 8;
 
     Type *new_array = nullptr;