Use the TextureHelper class to abstractly handle 2D and 3d textures.

Also refactor the Image11 copy methods to be a bit cleaner and not
use the copy conversion path when unnecessary.

This patch does not yet fix layer attachments - the fix for that will
come up in a subsequent patch.

BUG=angleproject:1290

Change-Id: If8b7aa8848ca4260e0dde690e7a99e115a97fabb
Reviewed-on: https://chromium-review.googlesource.com/323442
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>