Vulkan: Implement "default" vertex attributes. Rendering from disabled attributes is implemented using small dynamic buffers. The buffers use a stride of zero so the same vertex data is pulled for every index. This fixes all the disable attribute tests in dEQP. Bug: angleproject:2444 Change-Id: I04fe139076da3e3ff723bed8eb17e333b4cb0ddf Reviewed-on: https://chromium-review.googlesource.com/1136664 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>