D3D11: Add a TextureHelper class to wrap 2D+3D reads. 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/322680 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tryjob-Request: Jamie Madill <jmadill@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>