[subset] In the preprocess subset call always use long loca.

Long loca is needed so that we can store the trimmed glyph bytes to allow us to safely skip trimming in the later subset.

src/OT/glyf/glyf.hh

@@ -90,9 +90,12 @@ struct glyf
     | hb_map (&glyf_impl::SubsetGlyph::padded_size)
     ;
 
-    unsigned max_offset = + padded_offsets | hb_reduce (hb_add, 0);
-    bool use_short_loca = max_offset < 0x1FFFF;
-
+    bool use_short_loca = false;
+    if (likely (!c->plan->force_long_loca))
+    {
+      unsigned max_offset = + padded_offsets | hb_reduce (hb_add, 0);
+      use_short_loca = max_offset < 0x1FFFF;
+    }
 
     glyf_prime->serialize (c->serializer, glyphs.writer (), use_short_loca, c->plan, font);
     if (!use_short_loca) {

src/hb-subset-input.cc

@@ -489,6 +489,11 @@ hb_subset_preprocess (hb_face_t *source)
                             HB_SUBSET_FLAGS_RETAIN_GIDS);
   input->attach_accelerator_data = true;
 
+  // Always use long loca in the preprocessed version. This allows
+  // us to store the glyph bytes unpadded which allows the future subset
+  // operation to run faster by skipping the trim padding step.
+  input->force_long_loca = true;
+
   hb_face_t* new_source = hb_subset_or_fail (source, input);
   hb_subset_input_destroy (input);
 

src/hb-subset-input.hh

@@ -60,6 +60,10 @@ struct hb_subset_input_t
 
   unsigned flags;
   bool attach_accelerator_data = false;
+
+  // If set loca format will always be the long version.
+  bool force_long_loca = false;
+
   hb_hashmap_t<hb_tag_t, float> *axes_location;
 #ifdef HB_EXPERIMENTAL_API
   hb_hashmap_t<unsigned, hb_bytes_t> *name_table_overrides;

src/hb-subset-plan.cc

@@ -883,6 +883,7 @@ hb_subset_plan_create_or_fail (hb_face_t	 *face,
   void* accel = hb_face_get_user_data(face, hb_subset_accelerator_t::user_data_key());
 
   plan->attach_accelerator_data = input->attach_accelerator_data;
+  plan->force_long_loca = input->force_long_loca;
   if (accel)
     plan->accelerator = (hb_subset_accelerator_t*) accel;
 

src/hb-subset-plan.hh

@@ -108,6 +108,7 @@ struct hb_subset_plan_t
   bool successful;
   unsigned flags;
   bool attach_accelerator_data = false;
+  bool force_long_loca = false;
 
   // For each cp that we'd like to retain maps to the corresponding gid.
   hb_set_t *unicodes;