[plt-scheme] image.ss: (line ...) only draws to the SouthEast

While playing around with images today, I realized that the given  
implementation of the LINE function in image.ss calls error when  
given negative values for the x or y parameters.

This implies that you can only pass non-negative values for x and y,  
which in turn implies that the LINE function can't draw lines that go  
"SouthWest."  You can manipulate the pinholes to get lines that go  
NorthWest, but SouthWest and NorthEast are impossible with LINE.

ADD-LINE is capable of drawing lines in any direction, but wouldn't  
it be good to fix LINE so that it does handle negative x or y (and  
keeps the pinhole at the origin)?  Or is there some reason for  
requiring non-negative values that I'm not seeing?

-Felix