Fix poor cache image quality on device with higher pixelRatio. 

Changes:

1. Get global Kinetic.pixelRatio back, it will be set through stage's config.
2. Always use the calculated or user specified pixelRatio to get better image quality.
3. Fixed Canvas.setPixelRatio. Take into account pixelRatio.
4. When drawing cached image, scales the image back to its original size.