Fill modes and values for transformation
Add a option to fill pixels outside of the resampled image differently.
What could be the options. Constant (Any float or NaN) or others (see https://scikit-image.org/docs/stable/api/skimage.transform.html#skimage.transform.warp mode options)
Main issue would be to represent this in the widget in a sensible manner.
This is also required to allow for a multi-stage registration as many backends don't like NaNs in their images.