Vulkan: Threaded monolithic pipeline creation With this change, once a pipeline is created out of libraries, a task is scheduled (if necessary) to asynchronously create a corresponding monolithic pipeline. Once the task is complete, the linked pipeline handle is replaced by the monolithic one, gaining back any performance that might have been lost due to the use of libraries. Bug: angleproject:7369 Change-Id: I525fb1e09f8bedc61b9dbef19f9cce7026ff9c53 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4031151 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com>