Vulkan: Add vkCmdFillBuffer support Implemented as part of an experiment. Allows easier buffer-related testing by providing a quick way of filling the buffer with dummy data. Bug: angleproject:3205 Change-Id: Ice8cfd0c2566c91a5fb10aaea57985d671d0e7b7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1665351 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tobin Ehlis <tobine@google.com>