Move GL resource handling enablers to QtGui.

Made resource handling more robust by attempting to free GL resources in
the correct thread, and not forcing a context to become current to free
resources.

Change-Id: Ie81d4005b608972375755571d9b50ce82080709b
Reviewed-on: http://codereview.qt.nokia.com/3258


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>