Merge pull request cocos2d#2722 from dingpinglv/Iss1507_addImageAsync

Fixed : Added addImageAsync to cc.textureCache.

Author: dingpinglv

cocos2d/core/textures/CCTextureCache.js

@@ -362,6 +362,7 @@ if (cc._renderType === cc._RENDER_TYPE_CANVAS) {
         return tex;
     };
 
+    _p.addImageAsync = _p.addImage;
     _p = null;
 
 } else {

cocos2d/core/textures/TexturesWebGL.js

@@ -901,5 +901,7 @@ cc._tmp.WebGLTextureCache = function () {
 
         return tex;
     };
-     _p = null;
+
+    _p.addImageAsync = _p.addImage;
+    _p = null;
 };