Use global context sharing from QtGui instead of QSGContext

This removes QSGContext::sharedOpenGLContext and replace its uses
with QOpenGLContextPrivate::globalShareContext, which is also going
to be used by QOpenGLWidget and QQuickWidget.

Change-Id: I1e296c3e6832f717caaf31ba7d7b27c06249219b
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>