Instead of using an Impl type for default attachments, store the
egl::Surface pointer where possible.

BUG=angleproject:963

Change-Id: I3e34849e8b1ccae0c91a79617ec6f64aaaab6b10
Reviewed-on: https://chromium-review.googlesource.com/263483Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>