Remove the fill padding that is present in CT images.

When performing registration with anything but Mattes mutual information,
which ignores everything outside of a chosen intensity range, the padding
outside of the CT reconstruction diameter was messing up the registration.
The padding value should be replaced by the background (air) CT value before
the registration begins.
Call the variable "showTargetMoving", not "moveTarget".

I just wanted to make it clear that this variable only impacts the way
that the images are displayed.  During the registration process, the
source image is always the one that is being resampled.