Fix variable mew-image-display-resize ignored

The variable `mew-image-display-resize` was ignored and did not performed.
Maybe it has been from mew-6.2.52 (2009/08/12).
(It is before from github initial import.)

Now, if the variable is non-nil,
the image that is larger than frame size will be displayed with fitting to frame size.
The image that is smaller than frame size will be displayed in size as it is.