[COLR,glyf] Use map instead of set for tracking current glyphs / layers

src/OT/Color/COLR/COLR.hh

diff --git a/src/OT/Color/COLR/COLR.hh b/src/OT/Color/COLR/COLR.hh
index 3618a02..012c381 100644
--- a/src/OT/Color/COLR/COLR.hh
+++ b/src/OT/Color/COLR/COLR.hh
@@ -69,8 +69,8 @@
   unsigned int palette_index;
   hb_color_t foreground;
   VarStoreInstancer &instancer;
-  hb_set_t current_glyphs;
-  hb_set_t current_layers;
+  hb_map_t current_glyphs;
+  hb_map_t current_layers;
   int depth_left = HB_MAX_NESTING_LEVEL;
   int edge_count = HB_COLRV1_MAX_EDGE_COUNT;
 

src/OT/glyf/Glyph.hh

diff --git a/src/OT/glyf/Glyph.hh b/src/OT/glyf/Glyph.hh
index 9117fe8..1aed3ae 100644
--- a/src/OT/glyf/Glyph.hh
+++ b/src/OT/glyf/Glyph.hh
@@ -293,7 +293,7 @@
 		   bool use_my_metrics = true,
 		   bool phantom_only = false,
 		   hb_array_t<int> coords = hb_array_t<int> (),
-		   hb_set_t *current_glyphs = nullptr,
+		   hb_map_t *current_glyphs = nullptr,
 		   unsigned int depth = 0,
 		   unsigned *edge_count = nullptr) const
   {
@@ -303,7 +303,7 @@
     if (unlikely (*edge_count > HB_GLYF_MAX_EDGE_COUNT)) return false;
     (*edge_count)++;
 
-    hb_set_t current_glyphs_stack;
+    hb_map_t current_glyphs_stack;
     if (current_glyphs == nullptr)
       current_glyphs = &current_glyphs_stack;
 

src/hb-ft-colr.hh

diff --git a/src/hb-ft-colr.hh b/src/hb-ft-colr.hh
index 2690316..1afbbbb 100644
--- a/src/hb-ft-colr.hh
+++ b/src/hb-ft-colr.hh
@@ -105,8 +105,8 @@
   FT_Color *palette;
   unsigned palette_index;
   hb_color_t foreground;
-  hb_set_t current_glyphs;
-  hb_set_t current_layers;
+  hb_map_t current_glyphs;
+  hb_map_t current_layers;
   int depth_left = HB_MAX_NESTING_LEVEL;
   int edge_count = HB_COLRV1_MAX_EDGE_COUNT;
 };

src/hb-map.hh

diff --git a/src/hb-map.hh b/src/hb-map.hh
index 42604ef..6ea4166 100644
--- a/src/hb-map.hh
+++ b/src/hb-map.hh
@@ -276,6 +276,11 @@
     uint32_t hash = hb_hash (key);
     return set_with_hash (std::move (key), hash, std::forward<VV> (value), overwrite);
   }
+  bool add (const K &key)
+  {
+    uint32_t hash = hb_hash (key);
+    return set_with_hash (key, hash, item_t::default_value ());
+  }
 
   const V& get_with_hash (const K &key, uint32_t hash) const
   {