Hotfix/pixelperfect collision (should resolve #769)

Taken a different path to improve performance on non-Flash targets (not relying on the `BlendMode.DIFFERENCE` trick).
Added in @crazysam optimizations (https://gist.github.com/crazysam/8304760).
BitmapData cache was giving problems so I leaved that out.

Well, it needs more testing/benchmarking (not too versed in HaxeFlixel to do it myself), but already seems to behave quite well using the "initial" @joonjoonjoon test (60 fps stable against the drop to 30-ish I got with the original version).