Qt 5.{1,2} flickering fix (updated)

Since issue is not fixed in Qt 5.2, I am reopening this.
This fix sets environment variable for Qt renderer [Qt bugtracker] [1]

~~This patch is temporary solution for Qt5.1 compatibility issues. It makes SDDM at least usable wit Qt5.1.~~
~~- Flickering: for some reason it resolves when screen size isn't equal to window size;~~
~~- No pointer/X pointer: seems like Qt5.1 doesn't set root window cursor anymore (not sure if its is bug or not).~~
~~So there are 2 ugly workarounds, however one can be improved, if we will properly change root cursor via xcb.~~

  [1]: https://bugreports.qt-project.org/browse/QTBUG-32415?focusedCommentId=221915&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-226044 "Qt bugreport"