BitmapData.threshold() for CPP target.

BitmapData.threshold() implementation for cpp target. Pretty fast! Also tested and accurate - same visuals and return values as Flash API version. Got lots of help from crazysam (Sam Batista) and the HaxeFlixel community on this one.

More details:
http://www.haxeflixel.com/forum/general-discussion/bitmapdatathreshold-palette-swap-solution-cpp-target
http://www.nme.io/community/forums/feature-requests/my-implementation-bitmapdatathreshold-cpp-target/#ccm-discussion-post-anchor

![nme_threshold_flash](https://f.cloud.github.com/assets/739064/353963/63400df8-a090-11e2-807b-c01db32ab29d.png)
![nme_threshold_cpp](https://f.cloud.github.com/assets/739064/353964/65529494-a090-11e2-8f0e-e8997b246ece.png)