Having a retina and non-retina screen connected resulted in getting
no output from QQuickRenderControl and QQuickWidget on the
non-retina screen.

This is caused by the fact that Quick is blindly calling
QWindow::devicePixelRatio(). This approach is wrong when using
QQuickRenderControl since the QQuickWindow does not have an actual
native window and so devicePixelRatio() merely returns some default value
which will definitely be wrong for one of the screens.

The patch fixes the problem by introducing
QQuickWindow::effectiveDevicePixelRatio(), which, via
QQuickRenderControl::renderWindowFor, supports the redirected case too.

Task-number: QTBUG-42114
Change-Id: I057e01f0a00758dde438fc9b10af3a435b06bb0b
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>