[sanitize/Coverage] Keep a map of sane coverages

Fonts like Gulzar reuse the same coverage over a thousand times
sometimes.

However, this doesn't speed up sanitize unfortunately. Looks
like calling Coverage::sanitize() is already very fast. We're
just doing A LOT of it.

The map slowed it down in fact. A set was even slower.

Going to revert.

src/OT/Layout/Common/Coverage.hh

@@ -62,18 +62,30 @@ struct Coverage
 #endif
   bool sanitize (hb_sanitize_context_t *c) const
   {
+    uintptr_t offset = (uintptr_t) this - (uintptr_t) c->start;
+    if (offset < HB_SET_VALUE_INVALID &&
+	c->sane_coverages.has ((hb_codepoint_t) offset))
+      return true;
+
+    bool ret;
+
     TRACE_SANITIZE (this);
     if (!u.format.sanitize (c)) return_trace (false);
     switch (u.format)
     {
-    case 1: return_trace (u.format1.sanitize (c));
+    case 1: ret = u.format1.sanitize (c); break;
-    case 2: return_trace (u.format2.sanitize (c));
+    case 2: ret = u.format2.sanitize (c); break;
 #ifndef HB_NO_BEYOND_64K
-    case 3: return_trace (u.format3.sanitize (c));
+    case 3: ret = u.format3.sanitize (c); break;
-    case 4: return_trace (u.format4.sanitize (c));
+    case 4: ret = u.format4.sanitize (c); break;
 #endif
-    default:return_trace (true);
+    default:ret = true; break;
     }
+
+    if (likely (offset < HB_SET_VALUE_INVALID && ret))
+      c->sane_coverages.set ((hb_codepoint_t) offset, 1);
+
+    return_trace (ret);
   }
 
   /* Has interface. */

src/hb-sanitize.hh

@@ -32,6 +32,7 @@
 #include "hb.hh"
 #include "hb-blob.hh"
 #include "hb-dispatch.hh"
+#include "hb-map.hh"
 
 
 /*
@@ -228,6 +229,7 @@ struct hb_sanitize_context_t :
     this->edit_count = 0;
     this->debug_depth = 0;
     this->recursion_depth = 0;
+    this->sane_coverages.clear ();
 
     DEBUG_MSG_LEVEL (SANITIZE, start, 0, +1,
 		     "start [%p..%p] (%lu bytes)",
@@ -505,6 +507,7 @@ struct hb_sanitize_context_t :
   bool  num_glyphs_set;
   public:
   bool lazy_some_gpos;
+  hb_map_t sane_coverages;
 };
 
 struct hb_sanitize_with_object_t