effects: Fix QQuickItem::mapToGlobal() in qtquick effects on wayland

On Wayland, we create a dummy window to fix HiDPI issues. At the moment,
the QQuickRenderControl::renderWindow() returns a fixed offset 0,0.

As it turns out, QQuickItem::mapToGlobal() will poke the
QQuickRenderControl::renderWindow() function to map the specified point
to the global coordinate space.

Since the renderWindow() function returns hardcoded offset, the
mapToGlobal() function is sort of broken in fullscreen effects.

In order to fix mapToGlobal() on Wayland, this change makes the
EffectQuickRenderControl return the position of the associated
OffscreenQuickView.