For OffscreenCanvas in workers, we need to run a separate RemoteGraphicsContextGLProxy instance for each worker.
Pull request: https://github.com/WebKit/WebKit/pull/6989
<rdar://problem/103089158>
Committed 257534@main (5b5498b400b8): <https://commits.webkit.org/257534@main> Reviewed commits have been landed. Closing PR #6989 and removing active labels.