Added original variable and fixed bugs creating subimages.

By using original data variable it will save time encoding again the
decoded data.
Since the new subimage will have different size, its data must be
decoded using the new size, for this reason it needs to use the
original, undecoded, data.
Fixed bugs with the ROI of sprite images

* Ncer.cs: Fixed problem reading rectangle area (signed values)

* Frame.cs: Fixed problem with the Frame ROI (Offset negative
  coordinates)

* Tests.cs: Added test to extract sprite images.