Vulkan: Allow gaps in vertex attributes by packing them. Work around restrictions with setting null vertex buffers on command buffers by binding ranges of non-null buffers. Enable VertexAttributeTest on ES2/Vulkan BUG=angleproject:2792 BUG=angleproject:2797 Change-Id: Ief9db1e60c8c9045a4101abe859302d529decbcb Reviewed-on: https://chromium-review.googlesource.com/1194282 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>