Investigate WebXR with WebKit

[msub2]

WebKit has had WebXR in the source tree for some time now thanks to work from Igalia, and it looks like their OpenXR backend is using XR_MNDX_egl_enable for doing EGL-backed sessions, which is only available in Monado and SteamVR Linux. A long time ago I tried getting WebXR working on WebKit in Windows, not knowing about this detail, but I never actually tried to get it working on Linux. Might be worth looking into.

[msub2]

The primary upstream target for this would probably be GNOME Web/Epiphany

[mrxz]

Initially I did try to get WPE Webkit with the OpenXR backend working, but ran into some issue. Don't remember the specifics, but it was to the effect that compiling/launching with the flags needed for WebXR caused rendering/stability issues with any WebGL context. At the time it felt that Gecko and Chromium were more fruitful to pursue.

Also, XR_MNDX_egl_enable is only supported by the Monado runtime, not by SteamVR as far as I know.