ImageMagick support added

[Eric Wasylishen]

Hey,
I committed support for ImageMagick to gui yesterday, which adds SVG rendering 
plus support for a whole pile of other formats. 
(http://www.imagemagick.org/script/formats.php)

A few notes: 
 - Vector images are rasterized at whatever resolution ImageMagick decides on - 
it seems to work OK, but we could control it explicitly if needed, or render at 
multiple resolutions.
 - The alpha channel renders properly with SVG and raster formats, but not with 
ps/eps/pdf (which ImageMagick delegates to Ghostscript) - you get a white 
background. Ghostscript is too slow for interactive use anyway, IMHO.
 - DPI/point size are correctly read from formats that support it.
 - Multiple images are read from formats that support it.  If you call 
+imageRepsWithData: on an animated gif, it should return all of the frames.

Overall I think it turned out quite nicely, and should be handy for viewing 
obscure / old image files.

--Eric