Get the viewport to render beneath the Tauri webview

[Keavon]

We will be rendering the Tauri webview for the Graphite GUI—where a transparent hole is cut out of the viewport area—over the top of the natively rendered viewport. The OS compositor needs to allow us to draw first the viewport, then the webview, every frame. This needs to work on Windows, Mac, and Linux.

Currently, we are blocked by issues with the Wayland compositor (an X11 fallback might be a feasible mitigation) and Windows API. See:

• [bug] Flickering with raw display handle + winit + wgpu + tauri v2 + transparency tauri-apps/tauri#9220
• @TrueDoctor please add more issues for the pertinent ecosystem blockers and expand this with the state of affairs