[vector] Support shrinking storage if exact size provided

Only do it if requested size is less than quarter of allocated size.

This has massive benefit during CFF subset preprocessing.

src/hb-vector.hh

@@ -342,24 +342,39 @@ struct hb_vector_t
     if (unlikely (in_error ()))
       return false;
 
-    if (likely (size <= (unsigned) allocated))
+    unsigned int new_allocated;
+    if (exact)
+    {
+      /* If exact was specified, we allow shrinking the storage. */
+      size = hb_max (size, length);
+      if (size <= (unsigned) allocated &&
+	  size >= (unsigned) allocated >> 2)
 	return true;
 
-    /* Reallocate */
+      new_allocated = size;
+    }
+    else
+    {
+      if (likely (size <= (unsigned) allocated))
+	return true;
 
-    unsigned int new_allocated = exact ? size : allocated;
+      new_allocated = allocated;
       while (size > new_allocated)
 	new_allocated += (new_allocated >> 1) + 8;
+    }
+
+
+    /* Reallocate */
 
     Type *new_array = nullptr;
     bool overflows =
       (int) in_error () ||
-      (new_allocated < (unsigned) allocated) ||
+      (new_allocated < size) ||
       hb_unsigned_mul_overflows (new_allocated, sizeof (Type));
     if (likely (!overflows))
       new_array = realloc_vector (new_allocated);
 
-    if (unlikely (!new_array))
+    if (unlikely (new_allocated && !new_array))
     {
       allocated = -1;
       return false;
@@ -429,6 +444,8 @@ struct hb_vector_t
       return;
 
     shrink_vector (size);
+
+    alloc (size, true); /* To force shrinking memory if needed. */
   }