If non-null, require the image to have this width.
If null, the image will pick a size that best preserves its intrinsic aspect ratio.
It is strongly recommended that either both the width and the height be specified, or that the widget be placed in a context that sets tight layout constraints, so that the image does not change size as it loads. Consider using fit to adapt the image's rendering to fit the given width and height if the exact image dimensions are not known in advance.
final double width