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>