Add two members to vk::Format:
- vertex data copy function
- flag indicating if the function converts or not
Use the function when streaming vertex data so it gets converted if needed.
Add fallbacks for integer formats.  These formats will now work everywhere,
as long as they are in client memory, not a buffer object.
Adjust test expectations accordingly.

BUG=angleproject:2405

Change-Id: I677221219d933c35740633a0ab7694293e218177
Reviewed-on: https://chromium-review.googlesource.com/1084328Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>