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. Pass layer's config into ScenCanvas and HitCanvas, so that user can easily overwrite pixelRatio of a canvas through layer's config. (Currently, to overwrite pixelRatio, you need to call layer.canvas.setPixelRatio explicitly).
3. Always use the calculated or user specified pixelRatio to get better image quality.
4. When drawing cached image, scales the image back to its original size.