Vulkan: vertex attributes in client memory. Support vertex data stored in client memory passed to glVertexAttribPointer. Only GL_FLOAT data is supported at this time. Includes a simple test. BUG=angleproject:1683 Change-Id: I3bc0cdefe02b02c046b0e85822019a0f1762235e Reviewed-on: https://chromium-review.googlesource.com/425137 Commit-Queue: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>