When copying to and from the same texture, we need to reject only
the feedback loops formed with the same levels of the texture -
copying between different unrelated layers and levels is fine.

This change also fixes a couple bugs in our D3D11 CopyTexSubImage3D
implementation. We were missing some "!" operators, and we actually
would hit an ASSERT when trying to blit from a level of a 3D texture.

BUG=angleproject:1685

Change-Id: Id715bebafe8336cf8bb95d0d06275a8b95e522e1
Reviewed-on: https://chromium-review.googlesource.com/425494
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>