CL/VK: Implement flush & finish Bug: angleproject:8632 Change-Id: I139e463c0b1c947cee68e65c40503e52f01e988b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5406615 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Austin Annestrand <a.annestrand@samsung.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>