one mask or many (unioned)
Optional target?: "object" | "background"'object' (default) edits the masked object; 'background'
inverts so everything except the object is edited.
Optional width?: numberoptional canvas width; defaults to the first mask's.
Optional height?: numberoptional canvas height; defaults to the first mask's.
Combine one or more segmentation masks into a single mask Buffer suitable for
editImage()'smaskinput (white = edit region, black = keep).