Target includes the binding (DEPTH/STENCIL/COLOR), which is not useful
for many operations. Simplify this to just passing the mip/layer.

This allows us to stop using this internal struct in other classes.

BUG=angleproject:1635

Change-Id: Ic5a11781bf45fe7835437fa1e363c190b876d453
Reviewed-on: https://chromium-review.googlesource.com/469152Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>