Move the vertex buffer copying and conversion functions from the D3D back
end so the Vulkan back end can use them.  No functional change.

BUG=angleproject:2405

Change-Id: If3c04a6c0dc37c78c1779d8b5d3cece263e8a031
Reviewed-on: https://chromium-review.googlesource.com/1083817
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>