closed #2741: Modify LabelTTF's rendering from direct draw to pre-render
Closed 2741: Solved the problem that LabelTTF didn't work on mobile browser.
Closed #2741: remove the debug codes.
Closed #2741: fixed a bug of SpriteCanvas that loading texture doesn't work when calling initWithFile
Merge pull request #1102 from dingpinglv/Iss2741_RefactorLabelTTFDraw

closed #2741: Modify LabelTTF's rendering from direct draw to pre-render