Small change to speed up imshow()

I routinely plot fairly large datasets (~4million

    > points) using imshow but on my machine this takes
    > about 11 secs to complete. I went through the code
    > and made a couple of minor changes where the image
    > data would get clipped by vmax and vmin only if the
    > user supplied a vmax or vmin. The clipping is
    > unnecessary if the user does not supply these values
    > since vmax and vmin default to the max and min in
    > the image. I also replaced two successive
    > where(...) calls with a single clip(...) call and
    > that seems to have helped a tiny bit as well. This
    > change has been tested on 0.80 compiled with
    > Numeric, though I can't envision how this would
    > break anything. The profiler tells me that my plot
    > time has decreased from 11.8 sec to 7.2 sec. Hope
    > this helps.

Thanks Scott -- this simple optimization should help a lot with common
use cases. I've applied it in my local tree (hasn't made CVS yet
because there are other more significant changes I'm working on that I
can't commit yet).