Add a non-preserved mode to QOpenGLWidget and QQuickWidget

The framebuffer object contents is preserved between paint events. The behavior is matching QOpenGLWindow's partial update modes. This is fine.

However, most apps do not care about incremental drawing, they redraw everything on each paint. When this is true, the framebuffer's depth and stencil contents can be discarded (glDiscardFramebuffer) right after the paint event, while the color attachment can be discarded after composition.

To do this, we need to be sure however that the application is not relying on the previous contents in the next paint. Therefore this needs a flag, similar to QOpenGLWindow's UpdateBehavior.