Full support for using pack buffers as vertex buffers or other
binding points requires us to copy through the staging buffer.

BUG=angle:511

Change-Id: Ife0f5390a1a83dfbe0fa0b19db72f3a8b82b6b83
Reviewed-on: https://chromium-review.googlesource.com/198435Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>