Before this commit, when the wand select tool was used on a layer that
was moved, it would make the selection relative to the image, and not
relative to the layer. This commit fixes that issue.
Wand Selection tool uses similar logic to the Bucket Tool. Flood filling
and threshold calculations to determine the affected area just in this
case we do not set the pixels of the selected area, instead we use
those pixels to alter the selection mask.
In the future we can probably abstract out the shared flood logic so
both tools can share the code.
