Make IdentityCMaps more compact.

IdentityCMap uses an array to represent a 16-bit unsigned identity
function. This is very space-inefficient, and some files cause multiple
IdentityCMaps to be instantiated (e.g. the one from #4580 has 74).

This patch make the representation implicit.

When loading the PDF from issue #4580, this change reduces peak RSS from
~370 to ~280 MiB. It also improves overall speed on that PDF by ~30%,
going from 522 ms to 366 ms.

src/core/cmap.js

@@ -302,20 +302,69 @@ var CMap = (function CMapClosure() {
 
       return [0, 1];
     }
-
   };
   return CMap;
 })();
 
+// A special case of CMap, where the _map array implicitly has a length of
+// 65535 and each element is equal to its index.
 var IdentityCMap = (function IdentityCMapClosure() {
   function IdentityCMap(vertical, n) {
     CMap.call(this);
     this.vertical = vertical;
     this.addCodespaceRange(n, 0, 0xffff);
-    this.mapCidRange(0, 0xffff, 0);
   }
   Util.inherit(IdentityCMap, CMap, {});
 
+  IdentityCMap.prototype = {
+    addCodespaceRange: CMap.prototype.addCodespaceRange,
+
+    mapCidRange: function(low, high, dstLow) {
+      error('should not call mapCidRange');
+    },
+
+    mapBfRange: function(low, high, dstLow) {
+      error('should not call mapBfRange');
+    },
+
+    mapBfRangeToArray: function(low, high, array) {
+      error('should not call mapBfRangeToArray');
+    },
+
+    mapOne: function(src, dst) {
+      error('should not call mapCidOne');
+    },
+
+    lookup: function(code) {
+      return (isInt(code) && code <= 0xffff) ? code : undefined;
+    },
+
+    contains: function(code) {
+      return isInt(code) && code <= 0xffff;
+    },
+
+    forEach: function(callback) {
+      for (var i = 0; i <= 0xffff; i++) {
+        callback(i, i);
+      }
+    },
+
+    charCodeOf: function(value) {
+      return (isInt(value) && value <= 0xffff) ? value : -1;
+    },
+
+    getMap: function() {
+      // Sometimes identity maps must be instantiated, but it's rare.
+      var map = new Array(0x10000);
+      for (var i = 0; i <= 0xffff; i++) {
+        map[i] = i;
+      }
+      return map;
+    },
+
+    readCharCode: CMap.prototype.readCharCode
+  };
+
   return IdentityCMap;
 })();