Make sure that proxy and original images are scaled to same size before
applying offset or crop.

During testing, I discovered, that raw cache will lose information whether
this image was proxy or not. Because of this, proxy images will not create
this cache type. It would be fairly easy to implement this functionality for
cache, but I have decided to not do it now, because I did not want to pass yet
another mostly hard-coded bool flag to cache system. Since image is proxy, it
should be fast to read anyway.

In case of using offset property, code was modified to make sure we scale
image only once. I also tried to make code more readable and streamlined and
cleaned up surrounding functions a bit.