add offset to reading the wsi region from the slide image.

[sinberlin2]

This is needed for the P1000 images, which have a large amount of white background in the slide images. Without the offset, the stain vectors are computed across a part of the slide image that mostly contains white background area. Aperio images do not have white background and therefore the offset was not needed before.

Fixes #23

[AjeyPaiK]

I performed a quick test to compare the scaled size of the WSI before this PR and after this PR. They're equal indicating the test passes.

On the same line, I also performed a visual inspection on the normalized outputs. See below

Before Normalization	After Normalization