Allow input fields of type image to submit x,y values
even if no name attribute is set.
I didn't look this up to see whether or not it is
allowed by the official HTML spec, but i noted that
when most browsers submit a form from clicking on an
unnamed image element it sends values 'x' and 'y' as
opposed to <name>.x and <name>.y had the element been
I tried to be careful to check that the submit button
was an image before accepting an empty name.