Added reference to external OpenGL documentation needed in QtGui.
Added reference to external OpenGL documentation needed in QtGui.

Change-Id: I43eeb668f9a23cacfc3944897b7587dffd3aa731
Reviewed-by: Geir Vattekar <566b604d287edf88fa7389d1cb9015d99d71f27e@nokia.com>

kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc