HiDPI support in CCLabelTTF

[ntrrgc]

This patch adds built-in HiDPI support to text labels, so you don't have to double font size and scale them down manually in order to have crisp text rendering in high resolution mobile devices.

[pandamicro]

@ntrrgc Thanks for the contribution, it's a very intelligent solution ~

[1scaR1]

Mmm now labels returns incorrect contentSize of them.(On retina(MacBook or iPad) twice more then they are)

[ntrrgc]

Yep, I was worried of this kind of issues. Fixing that one would be a matter of overriding one more method.

On the other hand, I haven't checked affine transformations other than scaling (how many does cocos2D support?). Depending on how they are implemented, labels using them may break too, case in which we may have to come with another solution.

[pandamicro]

@1scaR1 nice catch, I'll fix that

@ntrrgc I think we should find a way to isolate the texture size with the transform

[ntrrgc]

Ideally the size of the texture and the unscaled logical size of the sprite should be allowed to be different to support different image depths.

[pandamicro]

@1scaR1 Issue fixed in new PR