Vulkan: Refactor AttachmentOpsArray. Will allow for more flexibly setting ops when we defer clears. Bug: angleproject:4517 Change-Id: I7d9116bc92e90eb41a1030fea242eadf1cc74562 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2165629 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>