Fix issue with highResolutionDisplay

Hi,

On my macbook pro retina, the drawToBitmapData function was a little bit broken.
It does not take into account the contentScaleFactor.
This change fix it on my laptop, but I'm not sure it does not introduce another bug as I have trouble understanding the subtleties of mContentScaleFactor versus mNativeStageContentScaleFactor in Starling.

Thanks,

Seb