Move load functions table to renderer layer.
We will be able to use it for the Vulkan formats table.
BUG=angleproject:1455
Change-Id: Iddc09fe560b06880af72a6fa8eb03fa2792e2f8c
Reviewed-on: https://chromium-review.googlesource.com/367454
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
