Also refactor the Renderer11::copyImage functions to share code.

BUG=angleproject:1384

Change-Id: I95f4266c0c9b67a13cba17797989fcc0b0d50c1d
Reviewed-on: https://chromium-review.googlesource.com/347248Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>