Remove the direct OpenGL code path in Qt Quick

Qt 5.14 ships with an opt-in QRhi-based rendering path in Qt Quick. See https://doc-snapshots.qt.io/qt5-5.14/qtquick-visualcanvas-scenegraph-renderer.html#rendering-via-the-qt-rendering-hardware-interface

In Qt 6.0 this should be made the default, and the direct OpenGL code path must be removed.

Note that this will trigger the need to refactor certain things that were done in 5.14 in order to maintain binary compatibility. For example, QSGMaterialRhiShader should no longer inherit from QSGMaterialShader (as the latter is removed), but this has implications on QSGMaterialShader.