Add support for PPOs to the Vulkan back end.

Bug: angleproject:3570
Change-Id: I5403456929847c185467b008d810f31ecfcb60cc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2072652
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>