Vulkan: Store current Serial in RendererVk. This gives a stronger ordering on serials than if they're acquired by the ContextVk. Part of the steps of implementing multithreaded GL on Vulkan. Implements a "globalFinish" method in RendererVk that is triggered on ContextVk destruction. This helped fixed some racy object deletion situations where the ContextVk could have queued work that uses deleted objects. Flush all the Contexts before destruction to avoid these hanging deleted objects. Bug: angleproject:2464 Change-Id: I244e9bbf6cd47b272c7cbca45b0fb1eb46d626fc Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1791268 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>