Protect against crashes with invalid openglContext()

1) Don't fail the assertion in the animator jobs if we don't have a context
2) Delegate the opengl context check in QQuickWindow::createTextureFromImage
   to the QSGContext

Change-Id: I1b248895dcd6db406f1af8866fd0052dd7564899
Reviewed-by: Lars Knoll <lars.knoll@digia.com>