In some cases in our code we had hard-coded ID3D11Texture2D, where
the code should handle 2D and 3D textures. This happens in ReadPixels,
where we have to handle 3D textures when the user binds a layer of a
3D textures with FramebufferTextureLayer.

This is a refactoring patch only, which makes the error cleaner.
Tests and the bug fix to come in a follow-up patch.

BUG=angleproject:1290

Change-Id: Ie1c293dead4d5b6b4dd6ce24ba2a9146619b141d
Reviewed-on: https://chromium-review.googlesource.com/322680Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>