Fix qtwindow (2.4)

Two QT highgui fixes.

1.  A long-standing TODO, where the pixel values displayed at high zoom were incorrect for resizeable windows, including not changing their ratio for free-ratio windows.
2.  Fix for http://code.opencv.org/issues/2449 (which, contrary to the issue status, is not actually done)

(PR https://github.com/Itseez/opencv/pull/909 rebased onto 2.4)