Replacing default.img by default image 'cdata'

[Pantxo Diribarne]

Hi,

Currently, the call to "image ()" without argument builds an image 
object using data loaded from file (default.img). For consistency with 
other graphics primitives (and for matlab compatibility if it is of any 
importance here), I think octave should use the default image "cdata" 
instead.

Do people have an opinion against this modification in image.m behavior?

Pantxo