[Reg: 5.5->5.6] Top level OpenGL widgets consumes too much CPU and GPU power

https://codereview.qt-project.org/#/c/139775/ provides performance regression for TLW OpenGL widgets.
https://codereview.qt-project.org/#/c/145172/ provides another performance regression for OpenGL widgets.

I present performance tests done with attached software (TLW QOpenGLWidget, 1920x1034, 60FPS, look into Environment section):

• without https://codereview.qt-project.org/#/c/139775/ : CPU: 2%, GPU: 2%
• with https://codereview.qt-project.org/#/c/139775/ : CPU: 16%, GPU: 7%
• with https://codereview.qt-project.org/#/c/145172/ : CPU: 30%, GPU: 7%

The same performance issues are on native OpenGL widgets which works since https://codereview.qt-project.org/#/c/139775/. I added some comments to this patch why it is so slow - unnecessary calls to some heavy-cost functions.

The test software can also run parented QOpenGLWidget to QWidget and the it works properly if it is not a native widget.

Please notice that test software displays only simple triangle at 60FPS on quite fast PC!