Fix screenshot

This fixes the "empty dialog" bug I reported in #783

It looks like the root cause here is that the QT event loop is starved by the main update loop in rviz. See https://code.google.com/p/qgtkstyle/issues/detail?id=85 and http://stackoverflow.com/questions/16240608/when-the-window-of-qfiledialoggetopenfilename-opening-the-program-has-unexpec for background.

PR against hydro-devel because it's the branch that I'm currently running. I suspect this will work in indigo as well, but I don't have a machine to test it on.