Compositors provide an MTLSharedEvent to be signalled at the completion of scene rendering. Now that ANGLE supports signalling external MTLSharedEvent (https://bugs.webkit.org/show_bug.cgi?id=243529), WebXROpaqueFramebuffer rendering should us this to signal when the compositor is free to start composition.
<rdar://problem/101212895>
Pull request: https://github.com/WebKit/WebKit/pull/5425
Committed 255613@main (2a4dc7e13fde): <https://commits.webkit.org/255613@main> Reviewed commits have been landed. Closing PR #5425 and removing active labels.