This lets us share objects (Textures/RBs/Surface) in the attachment
class. It will let us squash the attachment classes into one type,
which will in turn let us store them by-value, instead of by-pointer.

BUG=angleproject:963

Change-Id: Ia9a43dbc3b99475c00f6bc2ed5475deef55addc3
Reviewed-on: https://chromium-review.googlesource.com/263487Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>