Poor QOpenGLWidget performance when using native window and some other bugs

This patch: https://codereview.qt-project.org/#/c/139775 makes possible to use native window on QOpenGLWidget, great! But the performance is poor.
I compared it to QOpenGLWidget (not native) and QGLWidget. Results with 60FPS 720p video on QMPlay2:

• QGLWidget - of course, the fastest (~2% GPU, window size doesn't change this value),
• QOpenGLWidget (not native) - slower (~2% GPU on small window, ~4% GPU on full screen (1920x1080)),
• QOpenGLWidget (native window) - the slowest, ~3% GPU on small window, ~8% - 9% GPU on full screen (1920x1080).

QOpenGLWidget native window (probably which doesn't have child widgets) should have exactly the same performance as QGLWidget, then it can be a good replacement.

There is also other problem: QOpenGLWidget native window flickers during resizing.

The latest thing is documentation. I'm not good at OpenGL, but probably I found a mistake: "It does not perform any clearing.", but default update behaviour: "QOpenGLWidget::NoPartialUpdate" clears the buffer: "The framebuffer object is invalidated between frames with glDiscardFramebufferEXT if supported or a glClear" as I observed in my application.

I found that this patch: https://codereview.qt-project.org/#/c/140945/ has negative impact on windows which uses QOpenGLWidget (not native). It locks the application when window is moving! The application doesn't render and doesn't respond on any event during moving and a while after!