[scikits-image] Restore previous io plugins (#152)

[Stéfan van der Walt]

On Fri, Mar 2, 2012 at 10:26 AM, Tony Yu <tsyu80 at gmail.com> wrote:
> Hmm, another issue: Using ``imshow(..., plugin='qt')`` doesn't behave
> correctly (even after the PR above) because the ``show()`` function still
> uses the default plugin (i.e. not necessarily the qt plugin), so the window
> opened by ``imshow`` just disappears.

We could stick a use_plugin call inside of qt's imshow.  It's kind of
nasty to adjust global state this way, but should fix the problem in
the majority of cases.

Stéfan