Release GL resources of ShaderEffectSource while we still have GL.

When a Window element is used, the QML window item is deleted before
the ShaderEffectSource component and the deleteLater to delete the
texture gets handled too late. We now register for the
sceneGraphInvalidated signal and release resources synchronously
while shutting down.

Task-number: QTBUG-35294
Change-Id: Id83b669ddc16723551e5612264ccbad6d3a9bbcb
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Update changelog for change Icf323618 / QTBUG-35174

Change-Id: Idff1507fdce960ad7dabc2662bb273bc6103c1ca
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Revert "Load "@2x" images on high-dpi "retina" systems."

This reverts commit 707bbe5dea9d7398b205124a54422f2fafb6f151.

By itself this is completely broken and the image will 
display at twice the size. We need to add code to handle
the @2x image but revert for now to get sane behavior.

Change-Id: Iccdb26e9d19930b5d0ef4b6c7f596c5766f77a8a
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>