The headers are moving from QtGui to QtOpenGL. By avoiding the
qualification we can keep them compiling either way. Also, add
opengl-private to make the types available.
Also removed the QGraphicsRotation hack to get access to the projected
rotation function of QMatrix4x4. The function is public now.
Task-number: QTBUG-74409
Change-Id: I216e8ca09f8e247f96627b081308e3a57c55c29c
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>