This extends of the copy, read and write functions to cover depth and
stencil formats.

Refactoring change only.

Bug: angleproject:2673
Change-Id: I4b0b2f4cf8621051cacd95cdbd6d70f94ca612e2
Reviewed-on: https://chromium-review.googlesource.com/1147152Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>