ImageSiblings no longer inherit from RefCountObeject because they may be EGL
or GL objects and should handle their own deletion.

This can cause GL resources to outlive their contexts.  When the GL resource
is deleted, simply orphan the image.

BUG=angleproject:2668

Change-Id: I4a5c12bbe6e725f946209f9b48345a4097c9c91c
Reviewed-on: https://chromium-review.googlesource.com/1153601
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>