Ensure we do not regress when doing Quick on OpenGL on QRhi when it comes to OpenGL context resets

Context reset (loss) on OpenGL. What is needed here? How to verify we do not regress in cases that are important on Linux with NVIDIA for KDE in particualr? (lots of trouble in due to driver decision to lose resources on sleep/wake up, this NV-specific stuff is also hooked up to the context reset notification in the xcb plugin)

Patches are coming in from KDE even as of 5.14 to fix up various pieces in Qt to better handle the case when makeCurrent() fails and QOpenGLContext::isValid() then reports false. We must not regress in this respect in 6.0 when doing OpenGL via the QRhi.

The situation is somewhat convoluted on Windows as well. Consider https://bugreports.qt.io/browse/QTBUG-80257?focusedCommentId=508389&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-508389

and that's just OpenGL.