We can share the same pointer for the subject binding and the binding
pointer. This further allows us to optimize buffer re-binding. The
shared memory increases cache coherency and reduces the number of
instructions needed.

Bug: angleproject:2891
Change-Id: Id3162fa79de203f75989e7289ea02cb2ea1bec73
Reviewed-on: https://chromium-review.googlesource.com/c/1270217
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>