CL/VK: Add fillBuffer staging/transfer routine Bug: angleproject:377545840 Change-Id: I90f6896f96af25d7776a36595930b81af8c1141b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6237414 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Austin Annestrand <a.annestrand@samsung.com>