imshow without resampling in the ps backend.

A patch that enables drawing image in ps backend without resampling is
committed in r8035.
So, please test it if you're interested.

The raw image is to be used only when interpolation=="nearest" and
there is only one image.
While extending this to other backend such as pdf and svg should be
straight forward, I want to hear how others think about the overall
approach, e.g., api change and such. The current approach is to
minimize change in backends.

There are a few minor issues, whose solution is not clear to me.

* If there are multiple images (and the ps backend is used), the
images are resampled as they are compositted into a single image.

* The current solution forces not to resample whenever
interpolation=="nearest", I think this is generally good behavior.
However, on the highly extreme case of very high resolution of image
(e.g., image dpi > 1000 ?), it might be better if the image is
resampled (i.e., downsampled).

One option would be to introduce a new "resample" keyword in the
imshow command (which will become the attribute of the images).