QQuickWidget: Keep the offscreen quick window size in sync

QQuickWidget: Keep the offscreen quick window size in sync

The resizing logic has some faults: The size change is not always communicated
to the offscreen QQuickWindow. When hiding and showing a QQuickWidget we may
return early from the resize event handler, skipping the geometry change for
the offscreen window. This is wrong.

To make sure the sizes always match, set the geometry together with the
FBO creation instead. This is much more robust and guarantees that the
FBO size and the QQuickWindow size will not be out of sync.

Task-number: QTBUG-40517
Change-Id: I61ef3ad2d23dff4280dbf03b57c03c1aeda8dc91
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>