The center slice for a nine-patch image.
The region of the image inside the center slice will be stretched both horizontally and vertically to fit the image into its destination. The region of the image above and below the center slice will be stretched only horizontally and the region of the image to the left and right of the center slice will be stretched only vertically.
The stretching will be applied in order to make the image fit into the box specified by fit. When centerSlice is not null, fit defaults to BoxFit.fill, which distorts the destination image size relative to the image's original aspect ratio. Values of BoxFit which do not distort the destination image size will result in centerSlice having no effect (since the nine regions of the image will be rendered with the same scaling, as if it wasn't specified).
final Rect centerSlice