Implement out-of-process XRUI via iframes

[speigg]

https://forum.playcanvas.com/t/solved-multi-threaded-operations-in-webxr-editor-vs-published/24199/6

Out-of-process-iframes working in Oculus by adding Origin-Agent-Cluster to parent page response. Super cool! In theory this means we can move more of what ethereal is doing into a process less likely to affect frame rate, including DOM updates, hashing, serializing, indexeddb texture cache upload/retrieval, etc. Coordinating between the the 3D scene and the ethereal DOM becomes a bit trickier (limited to postMessage api), but might be worth the hassle.

https://web.dev/origin-agent-cluster/

• Origin-Agent-Cluster performance isolation header
• implement rasterization worker iframe
• adapt XRUI to perform rasterization in worker iframe