Fixed crashes in QGLTextureDestroyer.

QGLTextureDestroyer will try to make the context current on the GUI
thread, regardless of whether it is owned by another thread. Use
QOpenGLSharedResourceGuard instead which does the right thing and takes
shared contexts into account.

Task-number: QTBUG-31403
Change-Id: I1377f9284995a7ba5af32c85296eef152fc035c8
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Disable "QBackingStore::flush() called with non-exposed window" warning

This warning is pretty annoying and doesn't necessarily imply that an
application is not working properly.

Task-number: QTBUG-28613
Change-Id: Id0a2ebd91f9e4d59dce3e3e29637988d8e6175a9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>