Split the context-private part of the state cache In preparation for passing it directly to entry point implementations, ensuring no access to the share-group-accessible part. Bug: angleproject:8224 Change-Id: I705e6a8fb5204bab71caffff4dcb56d16c3d6e10 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4678782 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>