WebEngine in the QRhi-based world

We cannot do much about Chromium but the integration with widgets and Quick use OpenGL directly. Consider what this means in practice when Qt Quick is running via QRhi, and if we should migrate away from direct OpenGL usage in these integrations.

Regardless of what's being done, the ability to interoperate with Qt Quick on QRhi with the OpenGL backend must be ensured and tested. In addition, consider what we can do when Qt Qiick is running on another rhi backend, such as, D3D11 or Vulkan or Metal. (especially since Windows and macOS will likely not default to using OpenGL anymore, but rather D3D and Metal)